Sonic & Knuckles is a 1994 video game, published and developed by Sega, released for the Sega Genesis / Mega Drive. The game marked Knuckles' debut as a playable character. Originally, Sonic & Knuckles was ment to form one game with Sonic the Hedgehog 3, but due to time and budget constraints, Sega split the titles apart and instead equipped Sonic & Knuckles with a special "lock-on" technology that allows its cartridge to be combined with the cartridge of Sonic 3 to merge into the intended game, with that of Sonic 2 to make Knuckles playable in the game, and with that of the original Sonic the Hedgehog to create Blue Sphere, a bundle containing thousands of Special Stages from Sonic 3.
The game was highly acclaimed by critics. Sonic & Knuckles was rereleased for PC in 1997 (Bundled with Sonic 3, the lock-on version and Blue Sphere as the Sonic & Knuckles Collection), for the Wii in 2007 (Through the Virtual Console) and for Xbox 360 in 2009 (Through the Xbox Live Arcade). The combined game, Sonic 3 & Knuckles, is now available on Steam and is compatible with the Mega Drive & Genesis Classics launcher. It has also been included in several Sonic compilations for additional platforms, like the Sega Saturn, GameCube, Xbox, PlayStation 2, PlayStation 3 and Nintendo DS.