File with a .REZ extension contains resources – content of a game, such as different types of objects, maps, music and sounds or a net with textures. REZ files are used by the shooter-games based on the Lithtech game engine, developed by Monolith Productions and Microsoft.

Content of most of the REZ files can be modified with the lithrez.Exe tool, usually added to a game, or with a specifically downloaded Dragon UnPACKer software or the Game Extractor. It enables implementing substantial changes in the gameplay. REZ files are used in the gaming products of Monolith company, as in Aliens vs Predator or Noone Lives Forever series, or in the Sierra Entertainment productions Die Hard, Jarhead Games Marine Sharpshooter and many others.