Halo: Combat Evolved is a 2001 first-person shooter game developed by Bungie and published by Microsoft Game Studios. It was released as a launch game for Microsoft's Xbox video game console on November 15, 2001. The game was ported to Microsoft Windows and Mac OS X in 2003. It was later released as a downloadable Xbox Original for the Xbox 360. Halo is set in the twenty-sixth century, with the player assuming the role of the Master Chief, a cybernetically enhanced supersoldier. The Chief is accompanied by Cortana, an artificial intelligence. Players battle aliens as they attempt to uncover the secrets of the eponymous Halo, a ring-shaped artificial world.

Bungie began the development of what would eventually become Halo CE in 1997. Initially, the game was a real-time strategy game that morphed into a third-person shooter before becoming a first-person shooter. During development, Microsoft acquired Bungie and turned Halo into a launch game for its first video game console, the Xbox. Halo was a critical and commercial success and is often praised as one of the greatest video games ever made. The game's popularity led to labels such as "Halo clone" and "Halo killer", applied to games either similar to or anticipated to be better than it. Its sequel, Halo 2, was released for the original Xbox in 2004, and the game spawned a multi-billion-dollar multimedia franchise that incorporates games, books, toys, and films.

The game inspired and was used in the fan-created Red vs. Blue video series, which is credited as one of the first major successes of machinima (the technique of using real-time 3D engines, often from video games, to create animated films). More than six million copies had been sold worldwide by November 2005. A remake of the game's campaign, Halo: Combat Evolved Anniversary, was released for Xbox 360 by 343 Industries on the 10th anniversary of the original game's launch. Anniversary was re-released alongside the original competitive multiplayer as part of Halo: The Master Chief Collection in 2014.