Half-Life: Standard Software Development Kit 2.1

The Half-Life Standard SDK (Software Development Kit) contains all the resources you need to create multiplayer games and maps, as well as basic single-player game modifications (mods), for Half-Life. Version 2.0 of the Half-Life SDK contains dozens of new features, including access to Valve's new networking system and graphical user interface toolkit (V-GUI). For more information on creating Half-Life mods, visit the Half Life Editing Resource Center.