diff --git a/lib/oga/xml/tree_builder.rb b/lib/oga/xml/tree_builder.rb
index 1da6d52..32dfad7 100644
--- a/lib/oga/xml/tree_builder.rb
+++ b/lib/oga/xml/tree_builder.rb
@@ -1,14 +1,21 @@
 module Oga
   module XML
     ##
-    # Class for building a DOM tree of XML/HTML nodes.
+    # The TreeBuilder class turns an AST into a DOM tree. This DOM tree can be
+    # traversed by requesting child elements, parent elements, etc.
     #
-    # @!attribute [r] ast
-    #  @return [Oga::AST::Node]
+    # Basic usage:
+    #
+    #     builder = Oga::XML::TreeBuilder.new
+    #     ast     = s(:element, ...)
+    #
+    #     builder.process(ast) # => #<Oga::XML::Document ...>
     #
     class TreeBuilder < ::AST::Processor
-      attr_reader :ast
-
+      ##
+      # @param [Oga::AST::Node] node
+      # @return [Oga::XML::Document]
+      #
       def on_document(node)
         document = Document.new
         document.children = process_all(node)
@@ -20,10 +27,23 @@ module Oga
         return document
       end
 
+      ##
+      # @param [Oga::AST::Node] node
+      # @return [Oga::XML::Comment]
+      #
       def on_comment(node)
         return Comment.new(:text => node.children[0])
       end
 
+      ##
+      # Processes an `(element)` node and its child elements.
+      #
+      # An element can have a parent, previous and next element as well as a
+      # number of child elements.
+      #
+      # @param [Oga::AST::Node] node
+      # @return [Oga::XML::Element]
+      #
       def on_element(node)
         ns, name, attr, *children = *node
 
@@ -32,7 +52,7 @@ module Oga
         end
 
         if children
-          children = process_all(children)
+          children = process_all(children.compact)
         end
 
         element = Element.new(
@@ -42,38 +62,73 @@ module Oga
           :children   => children
         )
 
-        element.children.each_with_index do |child, index|
-          if index > 0
-            child.previous = element.children[index - 1]
-          end
-
-          if index + 1 <= element.children.length
-            child.next = element.children[index + 1]
-          end
-
-          child.parent = element
-        end
+        process_children(element)
 
         return element
       end
 
+      ##
+      # @param [Oga::AST::Node] node
+      # @return [Oga::XML::Text]
+      #
       def on_text(node)
         return Text.new(:text => node.children[0])
       end
 
+      ##
+      # @param [Oga::AST::Node] node
+      # @return [Oga::XML::Cdata]
+      #
       def on_cdata(node)
         return Cdata.new(:text => node.children[0])
       end
 
+      ##
+      # Converts a `(attributes)` node into a Hash.
+      #
+      # @param [Oga::AST::Node] node
+      # @return [Hash]
+      #
       def on_attributes(node)
         pairs = process_all(node)
 
         return Hash[pairs]
       end
 
+      ##
+      # @param [Oga::AST::Node] node
+      # @return [Array]
+      #
       def on_attribute(node)
         return *node
       end
+
+      private
+
+      ##
+      # Iterates over the child elements of an element and assigns the parent,
+      # previous and next elements. The supplied object is modified in place.
+      #
+      # @param [Oga::XML::Element] element
+      #
+      def process_children(element)
+        amount = element.children.length
+
+        element.children.each_with_index do |child, index|
+          prev_index = index - 1
+          next_index = index + 1
+
+          if index > 0
+            child.previous = element.children[prev_index]
+          end
+
+          if next_index <= amount
+            child.next = element.children[next_index]
+          end
+
+          child.parent = element
+        end
+      end
     end # TreeBuilder
   end # XML
 end # Oga
diff --git a/spec/oga/xml/tree_builder_spec.rb b/spec/oga/xml/tree_builder_spec.rb
new file mode 100644
index 0000000..6b7d66d
--- /dev/null
+++ b/spec/oga/xml/tree_builder_spec.rb
@@ -0,0 +1,174 @@
+require 'spec_helper'
+
+describe Oga::XML::TreeBuilder do
+  before do
+    @builder = described_class.new
+  end
+
+  context '#on_element' do
+    context 'simple elements' do
+      before do
+        node = s(:element, 'foo', 'p', nil, nil)
+        @tag = @builder.process(node)
+      end
+
+      example 'return a Element node' do
+        @tag.is_a?(Oga::XML::Element).should == true
+      end
+
+      example 'include the name of the element' do
+        @tag.name.should == 'p'
+      end
+
+      example 'include the namespace of the element' do
+        @tag.namespace.should == 'foo'
+      end
+    end
+
+    context 'elements with attributes' do
+      before do
+        node = s(
+          :element,
+          nil,
+          'p',
+          s(:attributes, s(:attribute, 'key', 'value')),
+          nil
+        )
+
+        @tag = @builder.process(node)
+      end
+
+      example 'include the name of the element' do
+        @tag.name.should == 'p'
+      end
+
+      example 'include the attributes' do
+        @tag.attributes.should == {'key' => 'value'}
+      end
+    end
+
+    context 'elements with parent elements' do
+      before do
+        node = s(:element, nil, 'p', nil, s(:element, nil, 'span', nil, nil))
+        @tag = @builder.process(node)
+      end
+
+      example 'set the parent element' do
+        @tag.children[0].parent.should == @tag
+      end
+    end
+
+    context 'elements with next elements' do
+      before do
+        node = s(
+          :element,
+          nil,
+          'p',
+          nil,
+          s(:element, nil, 'a', nil, nil),
+          s(:element, nil, 'span', nil, nil)
+        )
+
+        @tag = @builder.process(node)
+      end
+
+      example 'set the next element' do
+        @tag.children[0].next.should == @tag.children[1]
+      end
+
+      example 'do not set the next element for the last element' do
+        @tag.children[1].next.should == nil
+      end
+    end
+
+    context 'elements with previous elements' do
+      before do
+        node = s(
+          :element,
+          nil,
+          'p',
+          nil,
+          s(:element, nil, 'a', nil, nil),
+          s(:element, nil, 'span', nil, nil)
+        )
+
+        @tag = @builder.process(node)
+      end
+
+      example 'set the previous element' do
+        @tag.children[1].previous.should == @tag.children[0]
+      end
+
+      example 'do not set the previous element for the first element' do
+        @tag.children[0].previous.should == nil
+      end
+    end
+
+    context 'elements with child elements' do
+      before do
+        node = s(:element, nil, 'p', nil, s(:element, nil, 'span', nil, nil))
+        @tag = @builder.process(node)
+      end
+
+      example 'include the name of the element' do
+        @tag.name.should == 'p'
+      end
+
+      example 'include the child element' do
+        @tag.children[0].is_a?(Oga::XML::Element).should == true
+      end
+
+      example 'include the name of the child element' do
+        @tag.children[0].name.should == 'span'
+      end
+    end
+  end
+
+  context '#on_text' do
+    before do
+      node = s(:text, 'Hello')
+      @tag = @builder.process(node)
+    end
+
+    example 'return a Text node' do
+      @tag.is_a?(Oga::XML::Text).should == true
+    end
+
+    example 'include the text of the node' do
+      @tag.text.should == 'Hello'
+    end
+  end
+
+  context '#on_cdata' do
+    before do
+      node = s(:cdata, 'Hello')
+      @tag = @builder.process(node)
+    end
+
+    example 'return a Cdata node' do
+      @tag.is_a?(Oga::XML::Cdata).should == true
+    end
+
+    example 'include the text of the node' do
+      @tag.text.should == 'Hello'
+    end
+  end
+
+  context '#on_attributes' do
+    before do
+      @node = s(
+        :attributes,
+        s(:attribute, 'foo', 'bar'),
+        s(:attribute, 'baz', 'wat')
+      )
+    end
+
+    example 'return the attributes as a Hash' do
+      @builder.process(@node).should == {'foo' => 'bar', 'baz' => 'wat'}
+    end
+
+    example 'return an empty Hash by default' do
+      @builder.process(s(:attributes)).should == {}
+    end
+  end
+end