Day of Defeat: Source is a team-based first-person shooter multiplayer video game developed by Valve Corporation. Set in World War II, the game is an updated version of Day of Defeat, moving from the GoldSrc engine used by its predecessor to the Source engine. The game was released for Microsoft Windows on 26 September 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 US Army and the German 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.

Upon release, the game received a generally favorable reception, praised for its atmospheric and strategic gameplay and its graphics, audio work and overall presentation. However, the game was criticized for the lack of content in it at the time of release, although subsequent updates to the game have added new game modes and levels.

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