diff --git a/lib/oga/xpath/evaluator.rb b/lib/oga/xpath/evaluator.rb
index 4832120..ce491dc 100644
--- a/lib/oga/xpath/evaluator.rb
+++ b/lib/oga/xpath/evaluator.rb
@@ -626,6 +626,22 @@ module Oga
         return on_call_boolean(context, left) || on_call_boolean(context, right)
       end
 
+      ##
+      # Processes the `+` operator.
+      #
+      # This operator converts the left and right expressions to numbers and
+      # adds them together.
+      #
+      # @param [Oga::XPath::Node] ast_node
+      # @param [Oga::XML::NodeSet] context
+      # @return [Float]
+      #
+      def on_add(ast_node, context)
+        left, right = *ast_node
+
+        return on_call_number(context, left) + on_call_number(context, right)
+      end
+
       ##
       # Delegates function calls to specific handlers.
       #
diff --git a/spec/oga/xpath/evaluator/operators/add_operator.rb b/spec/oga/xpath/evaluator/operators/add_operator.rb
new file mode 100644
index 0000000..8340799
--- /dev/null
+++ b/spec/oga/xpath/evaluator/operators/add_operator.rb
@@ -0,0 +1,34 @@
+require 'spec_helper'
+
+describe Oga::XPath::Evaluator do
+  context 'add operator' do
+    before do
+      @document  = parse('<root><a>1</a><b>2</b></root>')
+      @evaluator = described_class.new(@document)
+    end
+
+    example 'add two numbers' do
+      @evaluator.evaluate('1 + 2').should == 3.0
+    end
+
+    example 'add a number and a string' do
+      @evaluator.evaluate('1 + "2"').should == 3.0
+    end
+
+    example 'add two strings' do
+      @evaluator.evaluate('"1" + "2"').should == 3.0
+    end
+
+    example 'add a number and a node set' do
+      @evaluator.evaluate('root/a + 2').should == 3.0
+    end
+
+    example 'add two node sets' do
+      @evaluator.evaluate('root/a + root/b').should == 3.0
+    end
+
+    example 'return NaN when trying to add invalid values' do
+      @evaluator.evaluate('"" + 1').should be_nan
+    end
+  end
+end