A BSP file is a map file used by games developed with one of the Quake game engines. It contains the layout information, objects, and resources for a level map. BSP files are used by many games, including Quake, Quake 2, Quake 3, Half-Life, Half-Life 2, Team Fortress, Counter-Strike, Counter-Strike: Condition-Zero, Portal, and Call of Duty: World at War.

BSP stands for "Binary Space Partitioning," which is a technique that breaks up intricate polygons into convex sets. This allows 3D maps to be rendered more quickly. BSP files are made up of "lumps," which are chunks of data that are defined in the file header. These include Entities, Nodes, Vertices, Planes, Leaves, Visibility, Faces, and Textures. BSP maps are compiled into a binary format. They can be compiled from .MAP files using tools such as Q3Map2 and Irrlicht. They may also be created using id Software programs such as GtkRadiant and DarkRadiant. Since BSP files are compiled from map editor projects, they typically are not modified directly. Some BSP map decompilers exist, such as Q3Map2, EntSpy, Vmex, and BSP2MAP, but they typically do not decompile BSP files correctly. Quake engines that use BSP files include id Tech 1 (Doom Engine), Quake Engine (the original Quake engine), id Tech 2 (Quake II Engine), id Tech 3 (used originally for Quake III Arena), and id Tech 4 (originally used for Doom 3). NOTE: Since Valve's Source Engine originated from the Quake engine, it also uses BSP files for maps. The Source Engine was used for developing games such as Half-Life 2 and Team Fortress 2.