From StrategyWiki, the free strategy guide and walkthrough wiki

Lithtech is a game engine which was initially developed by Monolith Productions in collaboration with Microsoft. Monolith later formed a separate company, LithTech Inc., to deal with further advancements of the engine technology and currently, after a change of its corporate identity, LithTech Inc. is known as Touchdown Entertainment. A number of different video game developers, including Monolith itself, have been using Lithtech to power their first-person shooter games, thus contributing to the success of the engine, establishing it as an alternative to other products, such as the Quake Engine and Unreal Engines. The Lithtech engine is continually being developed.