diff --git a/mlp/costs.py b/mlp/costs.py
index 351980b..bd103b3 100644
--- a/mlp/costs.py
+++ b/mlp/costs.py
@@ -1,17 +1,40 @@
 # -*- coding: utf-8 -*-
-"""Model costs."""
+"""Model costs.
+
+This module defines cost functions, with the aim of model training being to
+minimise the cost function given a set of inputs and target outputs. The cost
+functions typically measure some concept of distance between the model outputs
+and target outputs.
+"""
 
 import numpy as np
 
 
 class MeanSquaredErrorCost(object):
-    """
-    """
+    """Mean squared error cost."""
 
     def __call__(self, outputs, targets):
+        """Calculates cost function given a batch of outputs and targets.
+
+        Args:
+            outputs: Array of model outputs of shape (batch_size, output_dim).
+            targets: Array of target outputs of shape (batch_size, output_dim).
+
+        Returns:
+            Scalar cost function value.
+        """
         return 0.5 * np.mean(np.sum((outputs - targets)**2, axis=1))
 
     def grad(self, outputs, targets):
+        """Calculates gradient of cost function with respect to outputs.
+
+        Args:
+            outputs: Array of model outputs of shape (batch_size, output_dim).
+            targets: Array of target outputs of shape (batch_size, output_dim).
+
+        Returns:
+            Gradient of cost function with respect to outputs.
+        """
         return outputs - targets
 
     def __repr__(self):
@@ -19,14 +42,31 @@ class MeanSquaredErrorCost(object):
 
 
 class BinaryCrossEntropyCost(object):
-    """
-    """
+    """Binary cross entropy cost."""
 
     def __call__(self, outputs, targets):
+        """Calculates cost function given a batch of outputs and targets.
+
+        Args:
+            outputs: Array of model outputs of shape (batch_size, output_dim).
+            targets: Array of target outputs of shape (batch_size, output_dim).
+
+        Returns:
+            Scalar cost function value.
+        """
         return -np.mean(
             targets * np.log(outputs) + (1. - targets) * np.log(1. - ouputs))
 
     def grad(self, outputs, targets):
+        """Calculates gradient of cost function with respect to outputs.
+
+        Args:
+            outputs: Array of model outputs of shape (batch_size, output_dim).
+            targets: Array of target outputs of shape (batch_size, output_dim).
+
+        Returns:
+            Gradient of cost function with respect to outputs.
+        """
         return (1. - targets) / (1. - outputs) - (targets / outputs)
 
     def __repr__(self):
@@ -34,15 +74,32 @@ class BinaryCrossEntropyCost(object):
 
 
 class BinaryCrossEntropySigmoidCost(object):
-    """
-    """
+    """Binary cross entropy cost with logistic sigmoid applied to outputs."""
 
     def __call__(self, outputs, targets):
+        """Calculates cost function given a batch of outputs and targets.
+
+        Args:
+            outputs: Array of model outputs of shape (batch_size, output_dim).
+            targets: Array of target outputs of shape (batch_size, output_dim).
+
+        Returns:
+            Scalar cost function value.
+        """
         probs = 1. / (1. + np.exp(-outputs))
         return -np.mean(
             targets * np.log(probs) + (1. - targets) * np.log(1. - probs))
 
     def grad(self, outputs, targets):
+        """Calculates gradient of cost function with respect to outputs.
+
+        Args:
+            outputs: Array of model outputs of shape (batch_size, output_dim).
+            targets: Array of target outputs of shape (batch_size, output_dim).
+
+        Returns:
+            Gradient of cost function with respect to outputs.
+        """
         probs = 1. / (1. + np.exp(-outputs))
         return probs - targets
 
@@ -50,25 +107,31 @@ class BinaryCrossEntropySigmoidCost(object):
         return 'BinaryCrossEntropySigmoidCost'
 
 
-class BinaryAccuracySigmoidCost(object):
-    """
-    """
-
-    def __call__(self, outputs, targets):
-        return ((outputs > 0) == targets).mean()
-
-    def ___repr__(self):
-        return 'BinaryAccuracySigmoidCost'
-
-
 class CrossEntropyCost(object):
-    """
-    """
+    """Multi-class cross entropy cost."""
 
     def __call__(self, outputs, targets):
+        """Calculates cost function given a batch of outputs and targets.
+
+        Args:
+            outputs: Array of model outputs of shape (batch_size, output_dim).
+            targets: Array of target outputs of shape (batch_size, output_dim).
+
+        Returns:
+            Scalar cost function value.
+        """
         return -np.mean(np.sum(targets * np.log(outputs), axis=1))
 
     def grad(self, outputs, targets):
+        """Calculates gradient of cost function with respect to outputs.
+
+        Args:
+            outputs: Array of model outputs of shape (batch_size, output_dim).
+            targets: Array of target outputs of shape (batch_size, output_dim).
+
+        Returns:
+            Gradient of cost function with respect to outputs.
+        """
         return -targets / outputs
 
     def __repr__(self):
@@ -76,30 +139,35 @@ class CrossEntropyCost(object):
 
 
 class CrossEntropySoftmaxCost(object):
-    """
-    """
+    """Multi-class cross entropy cost with Softmax applied to outputs."""
 
     def __call__(self, outputs, targets):
+        """Calculates cost function given a batch of outputs and targets.
+
+        Args:
+            outputs: Array of model outputs of shape (batch_size, output_dim).
+            targets: Array of target outputs of shape (batch_size, output_dim).
+
+        Returns:
+            Scalar cost function value.
+        """
         probs = np.exp(outputs)
         probs /= probs.sum(-1)[:, None]
         return -np.mean(np.sum(targets * np.log(probs), axis=1))
 
     def grad(self, outputs, targets):
+        """Calculates gradient of cost function with respect to outputs.
+
+        Args:
+            outputs: Array of model outputs of shape (batch_size, output_dim).
+            targets: Array of target outputs of shape (batch_size, output_dim).
+
+        Returns:
+            Gradient of cost function with respect to outputs.
+        """
         probs = np.exp(outputs)
         probs /= probs.sum(-1)[:, None]
         return probs - targets
 
     def __repr__(self):
         return 'CrossEntropySoftmaxCost'
-
-
-class MulticlassAccuracySoftmaxCost(object):
-    """
-    """
-
-    def __call__(self, outputs, targets):
-        probs = np.exp(outputs)
-        return np.mean(np.argmax(probs, -1) == np.argmax(targets, -1))
-
-    def __repr__(self):
-        return 'MulticlassAccuracySoftmaxCost'