diff --git a/lib/oga/xpath/evaluator.rb b/lib/oga/xpath/evaluator.rb
index d906ae5..4ca6b58 100644
--- a/lib/oga/xpath/evaluator.rb
+++ b/lib/oga/xpath/evaluator.rb
@@ -1132,6 +1132,36 @@ module Oga
         return false
       end
 
+      ##
+      # Processes the `sum()` function call.
+      #
+      # This function call takes a node set, converts each node to a number and
+      # then sums the values.
+      #
+      # As an example, take the following XML:
+      #
+      #     <root>
+      #       <a>1</a>
+      #       <b>2</b>
+      #     </root>
+      #
+      # Using the expression `sum(root/*)` the return value would be `3.0`.
+      #
+      # @param [Oga::XML::NodeSet] context
+      # @param [Oga::XPath::Node] expression
+      # @return [Float]
+      #
+      def on_call_sum(context, expression)
+        nodes = process(expression, context)
+        sum   = 0.0
+
+        nodes.each do |node|
+          sum += node.text.to_f
+        end
+
+        return sum
+      end
+
       ##
       # Processes an `(int)` node.
       #
diff --git a/spec/oga/xpath/evaluator/calls/sum_spec.rb b/spec/oga/xpath/evaluator/calls/sum_spec.rb
new file mode 100644
index 0000000..dadaf43
--- /dev/null
+++ b/spec/oga/xpath/evaluator/calls/sum_spec.rb
@@ -0,0 +1,19 @@
+require 'spec_helper'
+
+describe Oga::XPath::Evaluator do
+  context 'sum() spec' do
+    before do
+      @document  = parse('<root><a>1</a><b>2</b></root>')
+      @evaluator = described_class.new(@document)
+    end
+
+    example 'return the sum of the <root> node' do
+      # The test of <root> is "12", which is then converted to a number.
+      @evaluator.evaluate('sum(root)').should == 12.0
+    end
+
+    example 'return the sum of the child nodes of the <root> node' do
+      @evaluator.evaluate('sum(root/*)').should == 3.0
+    end
+  end
+end