Halo is a military science fiction video game series and media franchise, originally developed by Bungie and currently managed and developed by Halo Studios (previously 343 Industries), part of Microsoft's Xbox Game Studios. The series launched in November 2001 with the first-person shooter video game Halo: Combat Evolved and its tie-in novel, The Fall of Reach. The latest major installment, Halo Infinite, was released in 2021. Spinoffs include real-time strategy and twin-stick shooter games.

Bungie began as a developer of computer games for the Macintosh platform, making games including Marathon 2: Durandal (1995). After the company was acquired by Microsoft in 2000, their in-progress game, which started life as a real-time strategy game, became Halo: Combat Evolved, a first-person shooter and exclusive launch title for Microsoft's Xbox video game console. Following the success of Halo, Bungie developed additional Halo sequels before and after regaining its independence from Microsoft in 2007. Microsoft established 343 Industries to oversee Halo going forward, producing games itself and in partnership with other studios. The developer was restructured and rebranded as Halo Studios in 2024, in conjunction with producing new games that would shift the series away from being developed on proprietary game technology, and towards being available on multiple platforms beyond Xbox and Windows going forward.

Halo: Combat Evolved was the Xbox's flagship "killer app" and cemented Microsoft as a major competitor in the video game console space, and its sequels pioneered online matchmaking, social features, and video game marketing. The games have sold more than 81 million copies worldwide. With more than $6 billion in franchise sales, Halo is one of the highest-grossing media franchises of all time, spanning novels, graphic novels, comic books, short films, animated films, feature films, fan-made short machinima animations and other licensed products.