File with an .UPK extension contains a pack of different types of files, for instance sounds, maps, 3D models, textures or settings used by the Unreal Engine 3. The can be loaded into a game for demand, in the moment when they are needed.

Many games are based on the free engine utilising the UPK files, such as BioShock, Mass Effect, Mirror's Edge or Unreal Tournament 3. UPK files can be created and edited with the UDK environment (Unreal Development Kit).