Day Of Defeat is a popular Half-Life modification, which resembles a multiplayer FPS set in the European war theatre of World War II and was later released as a full standalone game on Steam. The movie "Saving Private Ryan" served as the main inspiration for the mod. In one of the later updates, the movie's sound effects were added to the modification.

Players can choose between two teams: Axis and U.S. Army. At the beginning of the round, the teams spawn in their own spawn areas. The goal of either team is to complete their objectives the list of which depends on the map played and prevent the other team from completing theirs. The game ends if one of the teams completes all their objectives or when the time limit expires in which case the team with the most completed objectives wins.

The list of weaponry resembles those used in the World War II. The game features hard recoil and the players are unable to fire when running or jumping.

The list of maps offered on game's release consisted of 15 maps which varied in size and locations. The most notable examples are Omaha Beach and Operation Avalanche.

DoD was later ported on Source engine and released as Day Of Defeat: Source.