Unreal Engine 3 Development Kit (UDK), September 2011, is the publicly available "free for non-commercial use" version that appears to be identical to that used to build the Firaxis version of XCOM "Enemy Unknown" and "Enemy Within". Later versions of the UDK are NOT compatible with Firaxis XCOM.

See the official UDK distribution site [http://www.unrealengine.com/products/udk] for details and documentation. Additional material available on the Unreal Development Network (UDN) [http://udn.epicgames.com/Three/WebHome.html] web site.

This is an older version of the UDK, posted here to ensure availability to Firaxis XCOM mod creators. Distributed under terms of the Unreal Engine 3 UDK EULA [http://www.unrealengine.com/products/udk/udk-licensing-resources#eula].