Sonic the Hedgehog CD, commonly called Sonic CD, is a 1993 side-scrolling platform game published by Sega for the Sega CD. The story follows Sonic the Hedgehog as he attempts to save an extraterrestrial body, Little Planet, from Doctor Robotnik. As a Sonic the Hedgehog series platformer, Sonic runs and jumps through several themed levels while collecting rings and defeating robots. Sonic CD is distinguished from other Sonic games by its time travel feature, a key aspect to the story and gameplay. By traveling through time, players can access different versions of stages featuring alternate layouts, music, and graphics based on the time period.
The Sega CD's flagship game, Sonic CD was conceived as an enhanced port of Sonic the Hedgehog 2, but was reworked after lackluster sales of Sonic 2 in Japan. It was developed by Sonic Team—the team directed by Sonic designer Naoto Ohshima—and was designed to show off the technical capabilities of the Sega CD system. The game features the debuts of Amy Rose and Metal Sonic, and includes animated cutscenes produced by Toei Animation. Two soundtracks were composed for the game: the original score was composed by Naofumi Hataya and Masafumi Ogata, while the North American score was composed by members of the Sega Technical Institute.
Sonic CD is often called one of the best games in the Sonic series and the platform game genre. Reviewers praised its exceptional size and the time travel feature, which they felt added depth. The music was also praised, though some believed the game did not use the Sega CD's capabilities to its fullest. It sold over one million copies, making it the bestselling Sega CD game. The game was ported to Windows as part of the Sega PC brand in 1996, and to PlayStation 2 and GameCube as part of Sonic Gems Collection in 2005. A remastered version, developed by Christian Whitehead using the Retro Engine, was released for various platforms and mobile devices in 2011.