Day of Defeat: Source is a team-based online first-person shooter multiplayer video game developed by Valve Corporation. Set in World War II, the game is a complete remake of Day of Defeat. It was updated from the GoldSrc engine used by its predecessor to the Source engine, and a remake of the game models. The game was released for Microsoft Windows on September 26, 2005, distributed through Valve's online content delivery service Steam. Retail distribution of the game was handled by Electronic Arts.
The game was officially announced in February 2005. During the course of its development, Day of Defeat: Source progressed from being a straight conversion of Day of Defeat, to the alteration of certain aspects of the game's design and introduction of several new features. In addition, Day of Defeat: Source has been used by Valve to present new design features on the Source engine, such as high dynamic range rendering and cinematic effects. The game itself revolves around two teams, the U.S. Army and the Wehrmacht, each with access to six player classes, fighting in a variety of scenarios inspired by World War II engagements in the European Theatre of 1944.