Sonic the Hedgehog CD is a 1993 platform game developed by Sega for the Sega CD. As Sonic the Hedgehog, the player attempts to protect an extraterrestrial body, Little Planet, from Doctor Robotnik. Like other Sonic games, Sonic runs through themed levels while collecting rings and defeating robots. Sonic CD introduces time travel as a game mechanic. By traveling through time, players can access different versions of stages, featuring alternative layouts, music, and graphics. Sonic CD began as a port of the Sega Genesis game Sonic the Hedgehog (1991), but developed into a separate project. Led by Sonic's co-creator Naoto Ohshima, the developers sought to show off the technical capabilities of the Sega CD. Sonic CD features the debuts of the recurring Sonic characters Amy Rose and Metal Sonic, and includes animated cutscenes created by Toei Animation. Two soundtracks were composed: the one featured in the Japanese and European versions was composed by Naofumi Hataya and Masafumi Ogata, while the one in the North American version was composed by Spencer Nilsen, David Young, and Mark Crew. Sonic CD was released in late 1993. It received critical acclaim and is often called one of the best Sonic and platform games. Reviewers praised its size, music, and time travel feature, although some felt it did not fully use the Sega CD's capabilities. It sold over 1.5 million copies, making it the bestselling Sega CD game. Sonic CD was ported to Windows as part of the Sega PC label in 1996, and to the PlayStation 2 and GameCube as part of Sonic Gems Collection in 2005. A remake, developed by Christian Whitehead using the Retro Engine, was released for various platforms in 2011 and rereleased as part of the Sonic Origins compilation in 2022.