"The Unreal Development Kit is the free edition of Unreal Engine 3 that provides access to the award-winning 3D game engine and professional toolset used in blockbuster video game development, architectural visualization, mobile game development, 3D rendering, digital films and more."

The September 2011 version produces files with the same engine and build data as "cooked" Firaxis XCOM "Enemy Unknown" files.  It is posted on the Nexus website to ensure it remains available to mod creators.  Later versions of the UDK are NOT compatible with Firaxis versions of the product.

See [http://www.unrealengine.com/products/udk/udk-licensing-resources#eula] for the End User License Agreement (EULA) terms and conditions.  The UDK is "free for educational and non-commercial use".

See [http://www.unrealengine.com/products/udk] for information about the UDK.

See [http://udn.epicgames.com/Three/WebHome.html] for additional material and resources for use with the UDK.

Additional material related specifically to the Firaxis XCOM game can be found on the Nexus wiki [http://wiki.tesnexus.com/index.php/Category:XCOM].  Particulars for using the UDK are in the article "Modding with the UDK - XCOM:EU 2012" [http://wiki.tesnexus.com/index.php/Modding_with_the_UDK_-_XCOM:EU_2012].