diff --git a/beetsplug/replaygain.py b/beetsplug/replaygain.py
index ba57481ea..060563665 100644
--- a/beetsplug/replaygain.py
+++ b/beetsplug/replaygain.py
@@ -464,10 +464,19 @@ class GStreamerBackend(object):
 
 
 class AudioToolsBackend(Backend):
+    """ReplayGain backend that uses `Python Audio Tools
+    <http://audiotools.sourceforge.net/>`_ and its capabilities to read more
+    file formats and compute ReplayGain values using it replaygain module.
+    """
     def __init__(self, config):
         self._import_audiotools()
 
     def _import_audiotools(self):
+        """Checks if it's possible the necessary modules. There is no check on the
+        file formats at runtime.
+
+        :raises :exc:`ReplayGainError`: if the modules cannot be imported
+        """
         try:
             import audiotools
             import audiotools.replaygain
@@ -479,6 +488,13 @@ class AudioToolsBackend(Backend):
         self._mod_replaygain = audiotools.replaygain
 
     def open_audio_file(self, item):
+        """Open the file to read the PCM stream from the using ``item.path``
+
+        :return: the audiofile instance
+        :rtype: :class:`audiotools.AudioFile`
+        :raises :exc:`ReplayGainError`: if the file is not found or the file format is
+            not supported
+        """
         try:
             audiofile = self._mod_audiotools.open(item.path)
         except IOError:
@@ -493,6 +509,15 @@ class AudioToolsBackend(Backend):
         return audiofile
 
     def init_replaygain(self, audiofile, item):
+        """Returns an initialized :class:`audiotools.replaygain.ReplayGain`
+        instance, which requires the sample rate of the song(s) on which
+        the ReplayGain values will be computed. The item is passed in case
+        the sample rate is invalid to log the stored item sample rate.
+
+        :return: initialized replagain object
+        :rtype: :class:`audiotools.replaygain.ReplayGain`
+        :raises: :exc:`ReplayGainError` if the sample rate is invalid
+        """
         try:
             rg = self._mod_replaygain.ReplayGain(audiofile.sample_rate())
         except ValueError:
@@ -503,11 +528,23 @@ class AudioToolsBackend(Backend):
         return rg
 
     def compute_track_gain(self, items):
+        """Compute ReplayGain values for the requested items.
+
+        :return list: list of :class:`Gain` objects
+        """
         return [self._compute_track_gain(item) for item in items]
 
     def _compute_track_gain(self, item):
+        """Compute ReplayGain value for the requested item.
+
+        :rtype: :class:`Gain`
+        """
         audiofile = self.open_audio_file(item)
         rg = self.init_replaygain(audiofile, item)
+        # Each call to title_gain on a replaygain object return peak and gain
+        # of the track.
+        # Note that the method needs an audiotools.PCMReader instance that can
+        # be obtained from an audiofile instance.
         rg_track_gain, rg_track_peak = rg.title_gain(audiofile.to_pcm())
 
         log.debug(
@@ -521,6 +558,10 @@ class AudioToolsBackend(Backend):
         return Gain(gain=rg_track_gain, peak=rg_track_peak)
 
     def compute_album_gain(self, album):
+        """Compute ReplayGain values for the requested album and its items.
+
+        :rtype: :class:`AlbumGain`
+        """
         log.debug(
             u'Analysing album {0} - {1}'.format(
                 album.albumartist,
@@ -528,6 +569,9 @@ class AudioToolsBackend(Backend):
             )
         )
 
+        # The first item is taken and opened to get the sample rate to
+        # initialize the replaygain object. The object is used for all the
+        # tracks in the album to get the album values.
         item = list(album.items())[0]
         audiofile = self.open_audio_file(item)
         rg = self.init_replaygain(audiofile, item)
@@ -548,6 +592,8 @@ class AudioToolsBackend(Backend):
                 )
             )
 
+        # After getting the values for all tracks, it's possible to get the
+        # album values
         rg_album_gain, rg_album_peak = rg.album_gain()
         log.debug(
             u'ReplayGain for Album {0} - {1}: {2:.2f}, {3:.2f}'.format(