diff --git a/src/alsa_card.rs b/src/alsa_card.rs
index 9e3e47216..e42f55f41 100644
--- a/src/alsa_card.rs
+++ b/src/alsa_card.rs
@@ -1,3 +1,10 @@
+//! Alsa audio subsystem.
+//!
+//! This mod mainly defines the `AlsaCard` struct, which is the only data
+//! structure interacting directly with the alsa library.
+//! No other struct should directly interact with the alsa bindings.
+
+
 use alsa::card::Card;
 use alsa::mixer::SelemChannelId::*;
 use alsa::mixer::{Mixer, Selem, SelemId};
@@ -18,24 +25,54 @@ use support_alsa::*;
 
 
 #[derive(Clone, Copy, Debug)]
+/// An "external" alsa card event, potentially triggered by anything.
 pub enum AlsaEvent {
+    /// An error.
     AlsaCardError,
+    /// Alsa card is disconnected.
     AlsaCardDiconnected,
+    /// The values of the mixer changed, including mute state.
     AlsaCardValuesChanged,
 }
 
 
+/// A fairly high-level alsa card struct. We save some redundant
+/// information in order to access it more easily, in addition to
+/// some information that is not purely alsa related (like callbacks).
 pub struct AlsaCard {
     _cannot_construct: (),
+    /// The raw alsa card.
     pub card: Card,
+    /// The raw mixer.
     pub mixer: Mixer,
+    /// The simple element ID. `Selem` doesn't implement the Copy trait
+    /// so we save the ID instead and can get the `Selem` by lookup.
     pub selem_id: SelemId,
+    /// Watch IDs from polling the alsa card. We need them when we
+    /// drop the card, so we can unregister the polling.
     pub watch_ids: Cell<Vec<u32>>,
+    /// Callback for the various `AlsaEvent`s.
     pub cb: Rc<Fn(AlsaEvent)>,
 }
 
 
 impl AlsaCard {
+    /// Create a new alsa card. Tries very hard to get a valid, playable
+    /// card and mixer, so this is not a 'strict' function.
+    /// ## `card_name`
+    /// If a card name is provided, it will be tried. If `None` is provided
+    /// or the given card name does not exist or is not playable, any other
+    /// playable card is tried.
+    /// ## `elem_name`
+    /// If an elem name is provided, it will be tried. If `None` is provided
+    /// or the given elem name does not exist or is not playable, any other
+    /// playable elem is tried.
+    /// ## `cb`
+    /// Callback for the various `AlsaEvent`s.
+    ///
+    /// # Returns
+    ///
+    /// `Ok(Box<AlsaCard>)` on success, `Err(error)` otherwise.
     pub fn new(card_name: Option<String>,
                elem_name: Option<String>,
                cb: Rc<Fn(AlsaEvent)>)
@@ -100,11 +137,13 @@ impl AlsaCard {
     }
 
 
+    /// Get the name of the alsa card.
     pub fn card_name(&self) -> Result<String> {
         return self.card.get_name().from_err();
     }
 
 
+    /// Get the name of the channel.
     pub fn chan_name(&self) -> Result<String> {
         let n = self.selem_id
             .get_name()
@@ -113,11 +152,17 @@ impl AlsaCard {
     }
 
 
+    /// Get the `Selem`, looked up by the `SelemId`.
     pub fn selem(&self) -> Selem {
         return self.mixer.find_selem(&self.selem_id).unwrap();
     }
 
 
+    /// Get the current volume. The returned value corresponds to the
+    /// volume range and might need to be interpreted (such as converting
+    /// to percentage). This always gets
+    /// the volume of the `FrontRight` channel, because the seems to be
+    /// the safest bet.
     pub fn get_vol(&self) -> Result<i64> {
         let selem = self.selem();
         let volume = selem.get_playback_volume(FrontRight);
@@ -126,24 +171,36 @@ impl AlsaCard {
     }
 
 
+    /// Sets the volume of the current card configuration.
+    /// ## `new_vol`
+    /// The volume corresponding to the volume range of the `Selem`. This
+    /// might need to be translated properly first from other formats
+    /// (like percentage).
     pub fn set_vol(&self, new_vol: i64) -> Result<()> {
         let selem = self.selem();
         return selem.set_playback_volume_all(new_vol).from_err();
     }
 
 
+    /// Gets the volume range of the currently selected card configuration.
+    ///
+    /// # Returns
+    ///
+    /// `(min, max)`
     pub fn get_volume_range(&self) -> (i64, i64) {
         let selem = self.selem();
         return selem.get_playback_volume_range();
     }
 
 
+    /// Whether the current card configuration can be muted.
     pub fn has_mute(&self) -> bool {
         let selem = self.selem();
         return selem.has_playback_switch();
     }
 
 
+    /// Get the mute state of the current card configuration.
     pub fn get_mute(&self) -> Result<bool> {
         let selem = self.selem();
         let val = selem.get_playback_switch(FrontRight)?;
@@ -151,6 +208,9 @@ impl AlsaCard {
     }
 
 
+    /// Set the mute state of the current card configuration.
+    /// ## `mute`
+    /// Passing `true` here means the card will be muted.
     pub fn set_mute(&self, mute: bool) -> Result<()> {
         let selem = self.selem();
         /* true -> mute, false -> unmute */
@@ -159,6 +219,9 @@ impl AlsaCard {
     }
 
 
+    /// Watch the given alsa card poll descriptors and
+    /// return the corresponding watch IDs for saving
+    /// in the `AlsaCard` struct.
     fn watch_poll_descriptors(polls: Vec<pollfd>,
                               acard: &AlsaCard)
                               -> Vec<c_uint> {
@@ -186,6 +249,7 @@ impl AlsaCard {
     }
 
 
+    /// Unwatch the given poll descriptors.
     fn unwatch_poll_descriptors(watch_ids: &Vec<u32>) {
         for watch_id in watch_ids {
             unsafe {
@@ -197,19 +261,7 @@ impl AlsaCard {
 
 
 impl Drop for AlsaCard {
-    // call Box::new(x), transmute the Box into a raw pointer, and then
-    // std::mem::forget
-    //
-    // if you unregister the callback, you should keep a raw pointer to the
-    // box
-    //
-    // For instance, `register` could return a raw pointer to the
-    // Box + a std::marker::PhantomData with the appropriate
-    // lifetime (if applicable)
-    //
-    // The struct could implement Drop, which unregisters the
-    // callback and frees the Box, by simply transmuting the
-    // raw pointer to a Box<T>
+    /// Destructs the watch IDs corresponding to the current poll descriptors.
     fn drop(&mut self) {
         debug!("Destructing watch_ids: {:?}", self.watch_ids.get_mut());
         AlsaCard::unwatch_poll_descriptors(&self.watch_ids.get_mut());
@@ -217,6 +269,7 @@ impl Drop for AlsaCard {
 }
 
 
+/// The C callback function registered in `watch_poll_descriptors()`.
 extern "C" fn watch_cb(chan: *mut glib_sys::GIOChannel,
                        cond: glib_sys::GIOCondition,
                        data: glib_sys::gpointer)
diff --git a/src/app_state.rs b/src/app_state.rs
index c51f3a346..c0563f288 100644
--- a/src/app_state.rs
+++ b/src/app_state.rs
@@ -1,3 +1,6 @@
+//! Global application state.
+
+
 use audio::{Audio, AudioUser};
 use errors::*;
 use gtk;
@@ -12,17 +15,23 @@ use notif::*;
 
 
 // TODO: destructors
+/// The global application state struct.
 pub struct AppS {
     _cant_construct: (),
+    /// Mostly static GUI state.
     pub gui: Gui,
+    /// Audio state.
     pub audio: Audio,
+    /// Preferences state.
     pub prefs: RefCell<Prefs>,
     #[cfg(feature = "notify")]
+    /// Notification state.
     pub notif: Notif,
 }
 
 
 impl AppS {
+    /// Create an application state instance. There should really only be one.
     pub fn new() -> AppS {
         let builder_popup_window =
             gtk::Builder::new_from_string(include_str!(concat!(env!("CARGO_MANIFEST_DIR"),
@@ -59,6 +68,7 @@ impl AppS {
 
     /* some functions that need to be easily accessible */
 
+    /// Update the tray icon state.
     pub fn update_tray_icon(&self) -> Result<()> {
         debug!("Update tray icon!");
         return self.gui.tray_icon.update_all(&self.prefs.borrow(),
@@ -66,25 +76,30 @@ impl AppS {
                                              None);
     }
 
+    /// Update the Popup Window state.
     pub fn update_popup_window(&self) -> Result<()> {
         debug!("Update PopupWindow!");
         return self.gui.popup_window.update(&self.audio);
     }
 
     #[cfg(feature = "notify")]
+    /// Update the notification state.
     pub fn update_notify(&self) -> Result<()> {
         return self.notif.reload(&self.prefs.borrow());
     }
 
     #[cfg(not(feature = "notify"))]
+    /// Update the notification state.
     pub fn update_notify(&self) -> Result<()> {
         return Ok(());
     }
 
+    /// Update the audio state.
     pub fn update_audio(&self, user: AudioUser) -> Result<()> {
         return audio_reload(&self.audio, &self.prefs.borrow(), user);
     }
 
+    /// Update the config file.
     pub fn update_config(&self) -> Result<()> {
         let prefs = self.prefs.borrow_mut();
         return prefs.store_config();
diff --git a/src/audio.rs b/src/audio.rs
index 069f247c2..2a23c9c18 100644
--- a/src/audio.rs
+++ b/src/audio.rs
@@ -1,3 +1,13 @@
+#![allow(missing_docs)] // enums
+
+//! High-level audio subsystem.
+//!
+//! This is the middleman between the low-level audio backend (alsa),
+//! and the high-level ui code.
+//! This abstraction layer allows the high-level code to be completely unaware
+//! of the underlying audio implementation, may it be alsa or whatever.
+
+
 use alsa_card::*;
 use errors::*;
 use glib;
@@ -6,11 +16,13 @@ use std::cell::Ref;
 use std::cell::RefCell;
 use std::f64;
 use std::rc::Rc;
+use support_alsa::*;
 use support_audio::*;
 
 
 
 #[derive(Clone, Copy, Debug)]
+/// The volume level of the current audio configuration.
 pub enum VolLevel {
     Muted,
     Low,
@@ -20,6 +32,7 @@ pub enum VolLevel {
 }
 
 
+/// An audio user, used to determine from where a signal originated.
 #[derive(Clone, Copy, Debug)]
 pub enum AudioUser {
     Unknown,
@@ -30,6 +43,8 @@ pub enum AudioUser {
 }
 
 
+/// An audio signal. This will be used to connect callbacks to the
+/// audio system and react appropriately.
 #[derive(Clone, Copy, Debug)]
 pub enum AudioSignal {
     NoCard,
@@ -42,6 +57,7 @@ pub enum AudioSignal {
 
 
 #[derive(Clone)]
+/// Convenience struct to make handling this madness easier.
 pub struct Handlers {
     inner: Rc<RefCell<Vec<Box<Fn(AudioSignal, AudioUser)>>>>,
 }
@@ -64,16 +80,39 @@ impl Handlers {
 }
 
 
+/// High-level Audio struct, which could theoretically be backend
+/// agnostic.
 pub struct Audio {
     _cannot_construct: (),
+    /// The alsa card.
     pub acard: RefCell<Box<AlsaCard>>,
+    /// Last timestamp of an internal action we triggered, e.g.
+    /// by setting the volume or the mute state.
     pub last_action_timestamp: Rc<RefCell<i64>>,
+    /// A set of handlers that react to audio signals. We can
+    /// connect to these.
     pub handlers: Handlers,
+    /// The step at which to increase/decrease the volume.
+    /// This value is basically from the preferences.
     pub scroll_step: Cell<u32>,
 }
 
 
 impl Audio {
+    /// Create a new Audio instance. This tries very hard to get
+    /// a working configuration from the backend.
+    /// ## `card_name`
+    /// If a card name is provided, it will be tried. If `None` is provided
+    /// or the given card name does not exist or is not playable, any other
+    /// playable card is tried.
+    /// ## `elem_name`
+    /// If an elem name is provided, it will be tried. If `None` is provided
+    /// or the given elem name does not exist or is not playable, any other
+    /// playable elem is tried.
+    ///
+    /// # Returns
+    ///
+    /// `Ok(Audio)` on success, `Err(error)` otherwise.
     pub fn new(card_name: Option<String>,
                elem_name: Option<String>)
                -> Result<Audio> {
@@ -116,6 +155,10 @@ impl Audio {
     }
 
 
+    /// Switches the current alsa card. Behaves the same way in regards to
+    /// `card_name` and `elem_name` as the `Audio::new()` method.
+    /// ## `user`
+    /// Where the card switch originates from.
     pub fn switch_acard(&self,
                         card_name: Option<String>,
                         elem_name: Option<String>,
@@ -152,6 +195,7 @@ impl Audio {
     }
 
 
+    /// Current volume.
     pub fn vol(&self) -> Result<f64> {
         let alsa_vol = self.acard
             .borrow()
@@ -160,6 +204,8 @@ impl Audio {
     }
 
 
+    /// Current volume level, nicely usable for e.g. selecting from a set
+    /// of images.
     pub fn vol_level(&self) -> VolLevel {
         let muted = self.get_mute().unwrap_or(false);
         if muted {
@@ -176,6 +222,16 @@ impl Audio {
     }
 
 
+    /// Set the current volume.
+    /// ## `new_vol`
+    /// Set the volume to this value.
+    /// ## `user`
+    /// Where the card switch originates from.
+    /// ## `dir`
+    /// The "direction" of the volume change, e.g. is it a decrease
+    /// or increase. This helps with rounding problems.
+    /// ## `auto_unmute`
+    /// Whether to automatically unmute if the volume changes.
     pub fn set_vol(&self,
                    new_vol: f64,
                    user: AudioUser,
@@ -231,6 +287,10 @@ impl Audio {
     }
 
 
+    /// Increase the volume. The step to increasy by is taken from
+    /// `self.scroll_step`.
+    /// ## `user`
+    /// Where the card switch originates from.
     pub fn increase_vol(&self,
                         user: AudioUser,
                         auto_unmute: bool)
@@ -242,6 +302,10 @@ impl Audio {
     }
 
 
+    /// Decrease the volume. The step to decrease by is taken from
+    /// `self.scroll_step`.
+    /// ## `user`
+    /// Where the card switch originates from.
     pub fn decrease_vol(&self,
                         user: AudioUser,
                         auto_unmute: bool)
@@ -253,16 +317,19 @@ impl Audio {
     }
 
 
+    /// Whether the current audio configuration can be muted.
     pub fn has_mute(&self) -> bool {
         return self.acard.borrow().has_mute();
     }
 
 
+    /// Get the mute state of the current audio configuration.
     pub fn get_mute(&self) -> Result<bool> {
         return self.acard.borrow().get_mute();
     }
 
 
+    /// Set the mute state of the current audio configuration.
     pub fn set_mute(&self, mute: bool, user: AudioUser) -> Result<()> {
         let mut rc = self.last_action_timestamp.borrow_mut();
         *rc = glib::get_monotonic_time();
@@ -290,18 +357,24 @@ impl Audio {
     }
 
 
+    /// Toggle the mute state of the current audio configuration.
     pub fn toggle_mute(&self, user: AudioUser) -> Result<()> {
         let muted = self.get_mute()?;
         return self.set_mute(!muted, user);
     }
 
 
+    /// Connect a signal handler to the audio subsystem. This can
+    /// be done from anywhere, e.g. in the UI code to react to
+    /// certain signals. Multiple handlers for the same signals are fine,
+    /// they will be executed in order.
     pub fn connect_handler(&self, cb: Box<Fn(AudioSignal, AudioUser)>) {
         self.handlers.add_handler(cb);
     }
 }
 
 
+/// Invokes the registered handlers.
 fn invoke_handlers(handlers: &Vec<Box<Fn(AudioSignal, AudioUser)>>,
                    signal: AudioSignal,
                    user: AudioUser) {
@@ -320,6 +393,9 @@ fn invoke_handlers(handlers: &Vec<Box<Fn(AudioSignal, AudioUser)>>,
 }
 
 
+/// The callback for alsa events that is passed to the alsa subsystem.
+/// This is the bridge between low-level alsa events and "high-level"
+/// audio system signals.
 fn on_alsa_event(last_action_timestamp: &mut i64,
                  handlers: &Vec<Box<Fn(AudioSignal, AudioUser)>>,
                  alsa_event: AlsaEvent) {
diff --git a/src/errors.rs b/src/errors.rs
index f698f2802..1299157e5 100644
--- a/src/errors.rs
+++ b/src/errors.rs
@@ -1,3 +1,5 @@
+#![allow(missing_docs)]
+
 use alsa;
 use glib;
 use png;
diff --git a/src/glade_helpers.rs b/src/glade_helpers.rs
index 4697b099c..7bd353d3e 100644
--- a/src/glade_helpers.rs
+++ b/src/glade_helpers.rs
@@ -1,3 +1,5 @@
+#![allow(missing_docs)]
+
 #[macro_export]
 macro_rules! create_builder_item {
     ($sname:ident, $($element: ident: $ty: ty),+) => {
diff --git a/src/lib.rs b/src/lib.rs
index 73238aca2..64450fd30 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,3 +1,5 @@
+#![warn(missing_docs)]
+
 #![feature(alloc_system)]
 extern crate alloc_system;
 
diff --git a/src/notif.rs b/src/notif.rs
index 8a2a41032..525430a17 100644
--- a/src/notif.rs
+++ b/src/notif.rs
@@ -1,3 +1,9 @@
+//! The notification subsystem.
+//!
+//! This subsystem utilizes libnotify to send notifications as popups
+//! to the desktop.
+
+
 use app_state::*;
 use audio::*;
 use errors::*;
@@ -26,6 +32,8 @@ use ui_tray_icon::*;
 
 
 
+/// An expression of our notification system. Holds all the relevant information
+/// needed by Gtk+ callbacks to interact with libnotify.
 pub struct Notif {
     enabled: Cell<bool>,
     from_popup: Cell<bool>,
@@ -38,6 +46,7 @@ pub struct Notif {
 }
 
 impl Notif {
+    /// Create a new notification instance from the current preferences.
     pub fn new(prefs: &Prefs) -> Result<Self> {
         let notif = Notif {
             enabled: Cell::new(false),
@@ -54,6 +63,8 @@ impl Notif {
         return Ok(notif);
     }
 
+    /// Reload the notification instance from the current
+    /// preferences.
     pub fn reload(&self, prefs: &Prefs) -> Result<()> {
         let timeout = prefs.notify_prefs.notifcation_timeout;
 
@@ -74,6 +85,7 @@ impl Notif {
         return Ok(());
     }
 
+    /// Shows a volume notification, e.g. for volume or mute state change.
     pub fn show_volume_notif(&self, audio: &Audio) -> Result<()> {
         let vol = audio.vol()?;
         let vol_level = audio.vol_level();
@@ -114,6 +126,7 @@ impl Notif {
     }
 
 
+    /// Shows a text notification, e.g. for warnings or errors.
     pub fn show_text_notif(&self, summary: &str, body: &str) -> Result<()> {
         // TODO: error handling
         self.text_notif.update(summary, Some(body), None).unwrap();
@@ -126,6 +139,7 @@ impl Notif {
 
 
 
+/// Initialize the notification subsystem.
 pub fn init_notify(appstate: Rc<AppS>) {
     debug!("Blah");
     {
diff --git a/src/prefs.rs b/src/prefs.rs
index 7e6cfbe1a..acf600710 100644
--- a/src/prefs.rs
+++ b/src/prefs.rs
@@ -1,3 +1,12 @@
+#![allow(missing_docs)]
+
+//! The preferences subsystem.
+//!
+//! These are the global application preferences, which can be set
+//! by the user. They read from a file in TOML format, presented
+//! in the preferences dialog and saved back to the file on request.
+
+
 use errors::*;
 use std::fmt::Display;
 use std::fmt::Formatter;
@@ -18,6 +27,7 @@ const VOL_CONTROL_COMMANDS: [&str; 3] =
 
 #[derive(Deserialize, Debug, Serialize, Clone, Copy)]
 #[serde(rename_all = "snake_case")]
+/// When the tray icon is middle-clicked.
 pub enum MiddleClickAction {
     ToggleMute,
     ShowPreferences,
@@ -60,6 +70,7 @@ impl From<MiddleClickAction> for i32 {
 
 #[derive(Deserialize, Debug, Serialize)]
 #[serde(default)]
+/// Device preferences tab.
 pub struct DevicePrefs {
     pub card: String,
     pub channel: String, 
@@ -78,6 +89,7 @@ impl Default for DevicePrefs {
 
 #[derive(Deserialize, Debug, Serialize)]
 #[serde(default)]
+/// View preferences tab.
 pub struct ViewPrefs {
     pub draw_vol_meter: bool,
     pub vol_meter_offset: i32,
@@ -100,6 +112,7 @@ impl Default for ViewPrefs {
 
 #[derive(Deserialize, Debug, Serialize)]
 #[serde(default)]
+/// Volume color setting in the view preferences tab.
 pub struct VolColor {
     pub red: f64,
     pub green: f64,
@@ -119,6 +132,7 @@ impl Default for VolColor {
 
 #[derive(Deserialize, Debug, Serialize)]
 #[serde(default)]
+/// Behavior preferences tab.
 pub struct BehaviorPrefs {
     pub unmute_on_vol_change: bool,
     pub vol_control_cmd: Option<String>,
@@ -145,6 +159,7 @@ impl Default for BehaviorPrefs {
 #[cfg(feature = "notify")]
 #[derive(Deserialize, Debug, Serialize)]
 #[serde(default)]
+/// Notifications preferences tab.
 pub struct NotifyPrefs {
     pub enable_notifications: bool,
     pub notifcation_timeout: i64,
@@ -170,6 +185,7 @@ impl Default for NotifyPrefs {
 
 #[derive(Deserialize, Debug, Serialize, Default)]
 #[serde(default)]
+/// Main preferences struct, holding all sub-preferences.
 pub struct Prefs {
     pub device_prefs: DevicePrefs,
     pub view_prefs: ViewPrefs,
@@ -207,6 +223,7 @@ impl Prefs {
     }
 
 
+    /// Reload the current preferences from the config file.
     pub fn reload_config(&mut self) -> Result<()> {
         debug!("Reloading config...");
 
@@ -217,6 +234,7 @@ impl Prefs {
     }
 
 
+    /// Store the current preferences to the config file.
     pub fn store_config(&self) -> Result<()> {
         let config_path = get_xdg_dirs().place_config_file("pnmixer.toml")
             .from_err()?;
@@ -230,11 +248,16 @@ impl Prefs {
     }
 
 
+    /// Conver the current preferences to a viewable String.
     pub fn to_str(&self) -> String {
         return toml::to_string(self).unwrap();
     }
 
 
+    /// Get an available volume control command, which must exist in `$PATH`.
+    /// Tries hard to fine one,
+    /// starting with the given preference setting and falling back to the
+    /// `VOL_CONTROL_COMMANDS` slice.
     pub fn get_avail_vol_control_cmd(&self) -> Option<String> {
         match self.behavior_prefs.vol_control_cmd {
             Some(ref c) => return Some(c.clone()),
@@ -261,6 +284,7 @@ impl Display for Prefs {
 }
 
 
+/// Get the set of XDG directories, relative to our project.
 fn get_xdg_dirs() -> xdg::BaseDirectories {
     return xdg::BaseDirectories::with_prefix("pnmixer-rs").unwrap();
 }
diff --git a/src/support_alsa.rs b/src/support_alsa.rs
index 1fb85c95a..60635448c 100644
--- a/src/support_alsa.rs
+++ b/src/support_alsa.rs
@@ -1,3 +1,9 @@
+//! Alsa audio helper functions.
+//!
+//! This mod wraps around a few low-level alsa functions and abstracts
+//! out the details we don't care about.
+
+
 use alsa::card::Card;
 use alsa::mixer::{Mixer, Selem, SelemId, Elem};
 use alsa;
@@ -8,21 +14,25 @@ use std::iter::Filter;
 
 
 
+/// Get the default alsa card. This is the one with the ID `0`.
 pub fn get_default_alsa_card() -> Card {
     return get_alsa_card_by_id(0);
 }
 
 
+/// Get an alsa card corresponding to the given ID.
 pub fn get_alsa_card_by_id(index: c_int) -> Card {
     return Card::new(index);
 }
 
 
+/// Get all available alsa cards.
 pub fn get_alsa_cards() -> alsa::card::Iter {
     return alsa::card::Iter::new();
 }
 
 
+/// Get the first playable alsa card.
 pub fn get_first_playable_alsa_card() -> Result<Card> {
     for m_card in get_alsa_cards() {
         match m_card {
@@ -39,6 +49,7 @@ pub fn get_first_playable_alsa_card() -> Result<Card> {
 }
 
 
+/// Get the names of all playable alsa cards.
 pub fn get_playable_alsa_card_names() -> Vec<String> {
     let mut vec = vec![];
     for m_card in get_alsa_cards() {
@@ -59,6 +70,7 @@ pub fn get_playable_alsa_card_names() -> Vec<String> {
 }
 
 
+/// Get an alsa card by the given name.
 pub fn get_alsa_card_by_name(name: String) -> Result<Card> {
     for r_card in get_alsa_cards() {
         let card = r_card?;
@@ -71,6 +83,7 @@ pub fn get_alsa_card_by_name(name: String) -> Result<Card> {
 }
 
 
+/// Check whether the given alsa card as a playable `Selem`.
 pub fn alsa_card_has_playable_selem(card: &Card) -> bool {
     let mixer = try_wr!(get_mixer(&card), false);
     for selem in get_playable_selems(&mixer) {
@@ -82,11 +95,13 @@ pub fn alsa_card_has_playable_selem(card: &Card) -> bool {
 }
 
 
+/// Get the `Mixer` for the given alsa card.
 pub fn get_mixer(card: &Card) -> Result<Mixer> {
     return Mixer::new(&format!("hw:{}", card.get_index()), false).from_err();
 }
 
 
+/// Get the `Selem` from the given `Elem`.
 pub fn get_selem(elem: Elem) -> Selem {
     /* in the ALSA API, there are currently only simple elements,
      * so this unwrap() should be safe.
@@ -95,6 +110,7 @@ pub fn get_selem(elem: Elem) -> Selem {
 }
 
 
+/// Get all playable `Selem`s.
 pub fn get_playable_selems(mixer: &Mixer) -> Vec<Selem> {
     let mut v = vec![];
     for s in mixer.iter().map(get_selem).filter(selem_is_playable) {
@@ -104,6 +120,7 @@ pub fn get_playable_selems(mixer: &Mixer) -> Vec<Selem> {
 }
 
 
+/// Get the first playable `Selem`.
 pub fn get_first_playable_selem(mixer: &Mixer) -> Result<Selem> {
     for s in mixer.iter().map(get_selem).filter(selem_is_playable) {
         return Ok(s);
@@ -113,6 +130,7 @@ pub fn get_first_playable_selem(mixer: &Mixer) -> Result<Selem> {
 }
 
 
+/// Get the names from all playable `Selem`s.
 pub fn get_playable_selem_names(mixer: &Mixer) -> Vec<String> {
     let mut vec = vec![];
     for selem in get_playable_selems(mixer) {
@@ -127,6 +145,7 @@ pub fn get_playable_selem_names(mixer: &Mixer) -> Vec<String> {
 }
 
 
+/// Get a playable `Selem` by the given name.
 pub fn get_playable_selem_by_name(mixer: &Mixer,
                                   name: String)
                                   -> Result<Selem> {
@@ -143,6 +162,7 @@ pub fn get_playable_selem_by_name(mixer: &Mixer,
 }
 
 
+/// Check whether the given `Selem` is playable.
 pub fn selem_is_playable(selem: &Selem) -> bool {
     return selem.has_playback_volume();
 }
diff --git a/src/support_audio.rs b/src/support_audio.rs
index f6e0574b2..70ab25a2c 100644
--- a/src/support_audio.rs
+++ b/src/support_audio.rs
@@ -1,9 +1,18 @@
+#![allow(missing_docs)] // enum
+
+//! Helper functions of the audio subsystem.
+//!
+//! These functions are not directly connected to the `Audio` struct,
+//! but are important helpers.
+
+
 use audio::{Audio, AudioUser};
 use errors::*;
 use prefs::*;
 
 
 #[derive(Clone, Copy, Debug)]
+/// The direction of a volume change.
 pub enum VolDir {
     Up,
     Down,
@@ -11,6 +20,15 @@ pub enum VolDir {
 }
 
 
+/// Convert a volume change to the `VolDir` type.
+/// ## `old`
+/// The old volume value.
+/// ## `new`
+/// The new volume value.
+///
+/// # Returns
+///
+/// The direction of the volume change as `Voldir`.
 pub fn vol_change_to_voldir(old: f64, new: f64) -> VolDir {
     if old < new {
         return VolDir::Up;
@@ -22,6 +40,9 @@ pub fn vol_change_to_voldir(old: f64, new: f64) -> VolDir {
 }
 
 
+/// Kinda mimics `lrint` from libm. If the direction of the volume change
+/// is `Up` then calls `ceil()`, if it's `Down`, then calls `floor()`, otherwise
+/// returns the value unchanged.
 pub fn lrint(v: f64, dir: VolDir) -> f64 {
     match dir {
         VolDir::Up => v.ceil(),
@@ -30,6 +51,8 @@ pub fn lrint(v: f64, dir: VolDir) -> f64 {
     }
 }
 
+
+/// Reload the audio system.
 pub fn audio_reload(audio: &Audio,
                     prefs: &Prefs,
                     user: AudioUser)
@@ -41,6 +64,9 @@ pub fn audio_reload(audio: &Audio,
 }
 
 
+/// Converts the actual volume of the audio configuration, which depends
+/// on the volume range, to a scale of 0-100, reprenting the percentage
+/// of the volume level.
 pub fn vol_to_percent(vol: i64, range: (i64, i64)) -> Result<f64> {
     let (min, max) = range;
     ensure!(min < max,
@@ -52,6 +78,9 @@ pub fn vol_to_percent(vol: i64, range: (i64, i64)) -> Result<f64> {
 }
 
 
+/// Converts the percentage of the volume level (0-100) back to the actual
+/// low-level representation of the volume, which depends on the volume
+/// range.
 pub fn percent_to_vol(vol: f64, range: (i64, i64), dir: VolDir) -> Result<i64> {
     let (min, max) = range;
     ensure!(min < max,
diff --git a/src/support_cmd.rs b/src/support_cmd.rs
index 22be43157..ba3122c4d 100644
--- a/src/support_cmd.rs
+++ b/src/support_cmd.rs
@@ -1,3 +1,5 @@
+//! Helper functions for invoking system commands.
+
 use errors::*;
 use glib;
 use prefs::Prefs;
@@ -6,6 +8,9 @@ use std;
 
 
 
+/// Execute an available volume control command asynchronously, starting with
+/// the preferences and using some fallback values. If none of these
+/// are valid executables in `$PATH`, then return `Err(err)`.
 pub fn execute_vol_control_command(prefs: &Prefs) -> Result<()> {
     let m_cmd = prefs.get_avail_vol_control_cmd();
 
@@ -16,6 +21,7 @@ pub fn execute_vol_control_command(prefs: &Prefs) -> Result<()> {
 }
 
 
+/// Try to execute the given command asynchronously via gtk.
 pub fn execute_command(cmd: &str) -> Result<()> {
     return glib::spawn_command_line_async(cmd)
                .map_err(|e| {
diff --git a/src/support_ui.rs b/src/support_ui.rs
index 4dd77ea87..ed1793037 100644
--- a/src/support_ui.rs
+++ b/src/support_ui.rs
@@ -1,3 +1,6 @@
+//! Helper functions for the UI, mostly pixbuf functions.
+
+
 use errors::*;
 use gdk_pixbuf;
 use gdk_pixbuf_sys;
@@ -9,6 +12,8 @@ use std::path::*;
 
 
 
+/// Copy a `Pixbuf` explicitly, since they don't implement the `Copy` trait.
+/// Currently does not call `gdk_pixbuf_copy_options()`.
 pub fn copy_pixbuf(pixbuf: &gdk_pixbuf::Pixbuf) -> gdk_pixbuf::Pixbuf {
 
     let new_pixbuf = unsafe {
@@ -21,6 +26,8 @@ pub fn copy_pixbuf(pixbuf: &gdk_pixbuf::Pixbuf) -> gdk_pixbuf::Pixbuf {
 }
 
 
+/// Get a pixbuf by name from the given theme with the requested size.
+/// Note that the size is not enforced, but rather a hint.
 pub fn pixbuf_new_from_theme(icon_name: &str,
                              size: i32,
                              theme: &gtk::IconTheme)
@@ -42,6 +49,8 @@ pub fn pixbuf_new_from_theme(icon_name: &str,
 
 
 #[macro_export]
+/// Create a pixbuf from the given PNG file. Includes the file as bytes
+/// in the binary and decodes it.
 macro_rules! pixbuf_new_from_file {
     ($name:expr) => {
         {
diff --git a/src/ui_entry.rs b/src/ui_entry.rs
index 7723ed594..d5b4b93f5 100644
--- a/src/ui_entry.rs
+++ b/src/ui_entry.rs
@@ -1,3 +1,6 @@
+//! Global GUI state.
+
+
 use app_state::*;
 use audio::{AudioUser, AudioSignal};
 use gtk::DialogExt;
@@ -20,16 +23,23 @@ use notif::*;
 
 
 
+/// The GUI struct mostly describing the main widgets (mostly wrapped)
+/// the user interacts with.
 pub struct Gui {
     _cant_construct: (),
+    /// The tray icon.
     pub tray_icon: TrayIcon,
+    /// The popup window.
     pub popup_window: PopupWindow,
+    /// The popup menu.
     pub popup_menu: PopupMenu,
     /* prefs_dialog is dynamically created and destroyed */
+    /// The preferences dialog.
     pub prefs_dialog: RefCell<Option<PrefsDialog>>,
 }
 
 impl Gui {
+    /// Constructor. The prefs dialog is initialized as `None`.
     pub fn new(builder_popup_window: gtk::Builder,
                builder_popup_menu: gtk::Builder,
                prefs: &Prefs)
@@ -45,6 +55,7 @@ impl Gui {
 }
 
 
+/// Initialize the GUI system.
 pub fn init(appstate: Rc<AppS>) {
     {
         /* "global" audio signal handler */
@@ -79,6 +90,14 @@ pub fn init(appstate: Rc<AppS>) {
 }
 
 
+/// Used to run a dialog when an audio error occured, suggesting the user
+/// may reload the audio system either manually or by confirming the dialog
+/// via the confirmation button.
+///
+/// # Returns
+///
+/// `GTK_RESPONSE_YES` if the user wants to reload the audio system,
+/// `GTK_RESPONSE_NO` otherwise.
 fn run_audio_error_dialog(parent: &gtk::Window) -> i32 {
     error!("Connection with audio failed, you probably need to restart pnmixer.");
 
diff --git a/src/ui_popup_menu.rs b/src/ui_popup_menu.rs
index f906e7da5..767c654ff 100644
--- a/src/ui_popup_menu.rs
+++ b/src/ui_popup_menu.rs
@@ -1,3 +1,16 @@
+#![allow(missing_docs)] // glade_helpers
+
+//! The popup menu subsystem when the user right-clicks on the tray icon.
+//!
+//! Shows the menu with the following entries:
+//!
+//! * Mute
+//! * Volume Control
+//! * Preferences
+//! * Reload Sound
+//! * About
+//! * Quit
+
 use app_state::*;
 use audio::{AudioUser, AudioSignal};
 use gtk::prelude::*;
@@ -26,6 +39,7 @@ create_builder_item!(PopupMenu,
 
 
 
+/// Initialize the popup menu subsystem, registering all callbacks.
 pub fn init_popup_menu(appstate: Rc<AppS>) {
     /* audio.connect_handler */
     {
@@ -113,6 +127,7 @@ pub fn init_popup_menu(appstate: Rc<AppS>) {
 }
 
 
+/// When the about menu item is activated.
 fn on_about_item_activate(appstate: &AppS) {
     let popup_menu = &appstate.gui.popup_menu.menu_window;
     let about_dialog = create_about_dialog();
@@ -123,6 +138,7 @@ fn on_about_item_activate(appstate: &AppS) {
 }
 
 
+/// Create the About dialog from scratch.
 fn create_about_dialog() -> gtk::AboutDialog {
     let about_dialog: gtk::AboutDialog = gtk::AboutDialog::new();
 
@@ -153,12 +169,14 @@ Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.",
 }
 
 
+/// When the Preferences item is activated.
 fn on_prefs_item_activate(appstate: &Rc<AppS>) {
     /* TODO: only create if needed */
     show_prefs_dialog(appstate);
 }
 
 
+/// When the Mute item is checked.
 fn set_mute_check(apps: &Rc<AppS>) {
     let mute_check = &apps.gui.popup_menu.mute_check;
     let m_muted = apps.audio.get_mute();
diff --git a/src/ui_popup_window.rs b/src/ui_popup_window.rs
index ebc9af722..57136b9c8 100644
--- a/src/ui_popup_window.rs
+++ b/src/ui_popup_window.rs
@@ -1,3 +1,9 @@
+//! The popup window subsystem when the user left-clicks on the tray icon.
+//!
+//! This shows the manipulatable volume slider with the current volume and
+//! the mute checkbox.
+
+
 use app_state::*;
 use audio::*;
 use errors::*;
@@ -17,18 +23,30 @@ use support_cmd::*;
 
 
 
+/// The main struct for the popup window, holding all relevant sub-widgets
+/// and some mutable state.
 pub struct PopupWindow {
     _cant_construct: (),
+    /// The main window for the popup window widget.
     pub popup_window: gtk::Window,
+    /// The volume scale adjustment.
     pub vol_scale_adj: gtk::Adjustment,
+    /// The volume scale.
     pub vol_scale: gtk::Scale,
+    /// The mute check button.
     pub mute_check: gtk::CheckButton,
+    /// The button to start the external mixer.
     pub mixer_button: gtk::Button,
-    pub toggle_signal: Cell<u64>,
-    pub changed_signal: Cell<u64>,
+    /// Signal for mute_check.connect_toggled callback,
+    /// so we can block it temporarily.
+    toggle_signal: Cell<u64>,
+    /// Signal for vol_scale_adj.connect_value_changed callback,
+    /// so we can block it temporarily.
+    changed_signal: Cell<u64>,
 }
 
 impl PopupWindow {
+    /// Constructor.
     pub fn new(builder: gtk::Builder) -> PopupWindow {
         return PopupWindow {
                    _cant_construct: (),
@@ -42,6 +60,8 @@ impl PopupWindow {
                };
     }
 
+    /// Update the popup window state, including the slider
+    /// and the mute checkbutton.
     pub fn update(&self, audio: &Audio) -> Result<()> {
         let cur_vol = audio.vol()?;
         set_slider(&self.vol_scale_adj, cur_vol);
@@ -51,6 +71,7 @@ impl PopupWindow {
         return Ok(());
     }
 
+    /// Update the mute checkbutton.
     pub fn update_mute_check(&self, audio: &Audio) {
         let m_muted = audio.get_mute();
 
@@ -76,6 +97,8 @@ impl PopupWindow {
                                      self.toggle_signal.get());
     }
 
+    /// Set the page increment fro the volume scale adjustment based on the
+    /// preferences.
     fn set_vol_increment(&self, prefs: &Prefs) {
         self.vol_scale_adj
             .set_page_increment(prefs.behavior_prefs.vol_scroll_step);
@@ -85,7 +108,7 @@ impl PopupWindow {
 }
 
 
-
+/// Initialize the popup window subsystem.
 pub fn init_popup_window(appstate: Rc<AppS>) {
     /* audio.connect_handler */
     {
@@ -188,6 +211,7 @@ pub fn init_popup_window(appstate: Rc<AppS>) {
 }
 
 
+/// When the popup window is shown.
 fn on_popup_window_show(appstate: &AppS) {
     let popup_window = &appstate.gui.popup_window;
     appstate.gui.popup_window.set_vol_increment(&appstate.prefs.borrow());
@@ -201,6 +225,7 @@ fn on_popup_window_show(appstate: &AppS) {
 }
 
 
+/// On key or button press event on the popup window.
 fn on_popup_window_event(w: &gtk::Window, e: &gdk::Event) -> gtk::Inhibit {
     match gdk::Event::get_event_type(e) {
         gdk::EventType::GrabBroken => w.hide(),
@@ -230,6 +255,7 @@ fn on_popup_window_event(w: &gtk::Window, e: &gdk::Event) -> gtk::Inhibit {
 }
 
 
+/// When the volume scale slider is moved.
 fn on_vol_scale_value_changed(appstate: &AppS) {
     let audio = &appstate.audio;
     let old_vol = try_w!(audio.vol());
@@ -251,17 +277,20 @@ fn on_vol_scale_value_changed(appstate: &AppS) {
 }
 
 
+/// When the mute checkbutton is toggled.
 fn on_mute_check_toggled(appstate: &AppS) {
     let audio = &appstate.audio;
     try_w!(audio.toggle_mute(AudioUser::Popup))
 }
 
 
+/// Set the volume slider to the given value.
 pub fn set_slider(vol_scale_adj: &gtk::Adjustment, scale: f64) {
     vol_scale_adj.set_value(scale);
 }
 
 
+/// Grab all devices, keyboard and mouse.
 fn grab_devices(window: &gtk::Window) -> Result<()> {
     let device = gtk::get_current_event_device().ok_or("No current device")?;
 
diff --git a/src/ui_prefs_dialog.rs b/src/ui_prefs_dialog.rs
index 3b75a02ed..57bf1f477 100644
--- a/src/ui_prefs_dialog.rs
+++ b/src/ui_prefs_dialog.rs
@@ -1,3 +1,7 @@
+//! The preferences window subsystem, when the user clicks the "Preferences"
+//! menu item on the popup menu.
+
+
 use app_state::*;
 use audio::{AudioUser, AudioSignal};
 use errors::*;
@@ -11,6 +15,8 @@ use support_alsa::*;
 
 
 
+/// The main preferences dialog, holding all the relevant subwidgets we
+/// need to convert its state to preferences and back.
 pub struct PrefsDialog {
     _cant_construct: (),
     prefs_dialog: gtk::Dialog,
@@ -109,6 +115,7 @@ impl PrefsDialog {
     }
 
 
+    /// Import the given preferences into the preferences dialog state.
     fn from_prefs(&self, prefs: &Prefs) {
         /* DevicePrefs */
         /* filled on show signal with audio info */
@@ -171,6 +178,8 @@ impl PrefsDialog {
     }
 
 
+    /// Export the dialog state to the `Prefs` struct, which can be used
+    /// to write them to the config file.
     fn to_prefs(&self) -> Prefs {
         let card = self.card_combo.get_active_text();
         let channel = self.chan_combo.get_active_text();
@@ -242,6 +251,8 @@ impl PrefsDialog {
 }
 
 
+/// Show the preferences dialog. This is created and destroyed dynamically
+/// and not persistent across the application lifetime.
 pub fn show_prefs_dialog(appstate: &Rc<AppS>) {
     if appstate.gui
            .prefs_dialog
@@ -265,6 +276,8 @@ pub fn show_prefs_dialog(appstate: &Rc<AppS>) {
 }
 
 
+/// Initialize the internal prefs dialog handler that connects to the audio
+/// system.
 pub fn init_prefs_callback(appstate: Rc<AppS>) {
     let apps = appstate.clone();
     appstate.audio.connect_handler(Box::new(move |s, u| {
@@ -288,6 +301,7 @@ pub fn init_prefs_callback(appstate: Rc<AppS>) {
 }
 
 
+/// Initialize the preferences dialog gtk callbacks.
 fn init_prefs_dialog(appstate: &Rc<AppS>) {
 
     /* prefs_dialog.connect_show */
@@ -355,6 +369,7 @@ fn init_prefs_dialog(appstate: &Rc<AppS>) {
 }
 
 
+/// Fill the card combo box in the Devices tab.
 fn fill_card_combo(appstate: &AppS) {
     let m_cc = appstate.gui.prefs_dialog.borrow();
     let card_combo = &m_cc.as_ref().unwrap().card_combo;
@@ -381,6 +396,7 @@ fn fill_card_combo(appstate: &AppS) {
 }
 
 
+/// Fill the channel combo box in the Devices tab.
 fn fill_chan_combo(appstate: &AppS, cardname: Option<String>) {
     let m_cc = appstate.gui.prefs_dialog.borrow();
     let chan_combo = &m_cc.as_ref().unwrap().chan_combo;
diff --git a/src/ui_tray_icon.rs b/src/ui_tray_icon.rs
index 1cb9a0a96..67fc00e6d 100644
--- a/src/ui_tray_icon.rs
+++ b/src/ui_tray_icon.rs
@@ -1,3 +1,9 @@
+//! The tray icon subsystem.
+//!
+//! This manages the tray icon Pixbuf as well as the callbacks on left and
+//! right-click.
+
+
 use app_state::*;
 use audio::*;
 use errors::*;
@@ -19,16 +25,24 @@ use ui_prefs_dialog::show_prefs_dialog;
 const ICON_MIN_SIZE: i32 = 16;
 
 
+/// The tray icon struct, describing the complete visual state.
 pub struct TrayIcon {
     _cant_construct: (),
+    /// The volume meter to draw on the actual Pixbuf, if requested.
     pub volmeter: RefCell<Option<VolMeter>>,
+    /// The actual Pixbuf tray icon.
     pub audio_pix: RefCell<AudioPix>,
+    /// The gtk `StatusIcon` widget, used to register callbacks.
     pub status_icon: gtk::StatusIcon,
+    /// The current icon size.
     pub icon_size: Cell<i32>,
 }
 
 
 impl TrayIcon {
+    /// Constructor. `audio_pix` is initialized as empty GdkPixbuf, to save
+    /// one iteration of png decoding (`update_all()` is triggered immediately
+    /// on startup through `tray_icon.connect_size_changed`.
     pub fn new(prefs: &Prefs) -> Result<TrayIcon> {
         let draw_vol_meter = prefs.view_prefs.draw_vol_meter;
 
@@ -57,6 +71,8 @@ impl TrayIcon {
     }
 
 
+    /// Update the volume meter Pixbuf, which is drawn on top of the
+    /// actual Pixbuf.
     fn update_vol_meter(&self, cur_vol: f64, vol_level: VolLevel) -> Result<()> {
         let audio_pix = self.audio_pix.borrow();
         let pixbuf = audio_pix.select_pix(vol_level);
@@ -75,6 +91,7 @@ impl TrayIcon {
     }
 
 
+    /// Update the tooltip of the tray icon.
     fn update_tooltip(&self, audio: &Audio) {
         let cardname = audio.acard
             .borrow()
@@ -105,6 +122,7 @@ impl TrayIcon {
     }
 
 
+    /// Update the whole tray icon state.
     pub fn update_all(&self,
                       prefs: &Prefs,
                       audio: &Audio,
@@ -138,6 +156,8 @@ impl TrayIcon {
 
 
 
+/// The volume meter, describes by its colors, offset and width/row
+/// properties.
 pub struct VolMeter {
     red: u8,
     green: u8,
@@ -151,6 +171,7 @@ pub struct VolMeter {
 
 
 impl VolMeter {
+    /// Constructor. `width` and `row` are initialized with default values.
     fn new(prefs: &Prefs) -> VolMeter {
         return VolMeter {
                    red: (prefs.view_prefs.vol_meter_color.red * 255.0) as u8,
@@ -166,6 +187,7 @@ impl VolMeter {
     }
 
     // TODO: cache input pixbuf?
+    /// Draw the volume meter on top of the actual tray icon Pixbuf.
     fn meter_draw(&self,
                   volume: i64,
                   pixbuf: &gdk_pixbuf::Pixbuf)
@@ -246,6 +268,7 @@ impl VolMeter {
 
 
 #[derive(Clone, Debug)]
+/// The actual tray icon Pixbuf, which depends on the current volume level.
 pub struct AudioPix {
     muted: gdk_pixbuf::Pixbuf,
     low: gdk_pixbuf::Pixbuf,
@@ -277,6 +300,7 @@ impl Default for AudioPix {
 
 
 impl AudioPix {
+    /// Constructor.
     fn new(size: i32, prefs: &Prefs) -> Result<AudioPix> {
         let system_theme = prefs.view_prefs.system_theme;
 
@@ -336,6 +360,7 @@ impl AudioPix {
     }
 
 
+    /// Select the try icon Pixbuf depending on the `VolLevel`.
     fn select_pix(&self, vol_level: VolLevel) -> &gdk_pixbuf::Pixbuf {
         match vol_level {
             VolLevel::Muted => &self.muted,
@@ -348,6 +373,7 @@ impl AudioPix {
 }
 
 
+/// Initialize the tray icon subsystem.
 pub fn init_tray_icon(appstate: Rc<AppS>) {
     let tray_icon = &appstate.gui.tray_icon;
 
@@ -424,6 +450,7 @@ pub fn init_tray_icon(appstate: Rc<AppS>) {
 }
 
 
+/// When the tray icon is activated.
 fn on_tray_icon_activate(appstate: &AppS) {
     let popup_window = &appstate.gui.popup_window.popup_window;
 
@@ -435,6 +462,7 @@ fn on_tray_icon_activate(appstate: &AppS) {
 }
 
 
+/// When the popup menu is shown, hide the popup window, if any.
 fn on_tray_icon_popup_menu(appstate: &AppS) {
     let popup_window = &appstate.gui.popup_window.popup_window;
     let popup_menu = &appstate.gui.popup_menu.menu;
@@ -444,6 +472,8 @@ fn on_tray_icon_popup_menu(appstate: &AppS) {
 }
 
 
+/// When the mouse scroll event happens while the mouse pointer is
+/// on the tray icon.
 fn on_tray_icon_scroll_event(appstate: &AppS,
                              event: &gdk::EventScroll)
                              -> bool {
@@ -473,6 +503,9 @@ fn on_tray_icon_scroll_event(appstate: &AppS,
 }
 
 
+/// Basically when the tray icon is clicked (although we connect to the `release`
+/// event). This decides whether it was a left, right or middle-click and
+/// takes appropriate actions.
 fn on_tray_button_release_event(appstate: &Rc<AppS>,
                                 event_button: &gdk::EventButton)
                                 -> bool {