Sonic the Hedgehog 2 is a 1992 platform game developed by Sega Technical Institute (STI) for the Sega Genesis. It follows Sonic as he attempts to stop Doctor Robotnik from stealing the Chaos Emeralds to power his space station. Like the first Sonic the Hedgehog (1991), players traverse side-scrolling levels at high speeds while collecting rings, defeating enemies, and fighting bosses. Sonic 2 introduces Sonic's sidekick Miles "Tails" Prower and features faster gameplay, larger levels, a multiplayer mode, and special stages featuring pre-rendered 3D graphics.
Though the original Sonic the Hedgehog greatly increased the popularity of the Genesis in North America, Sega was slow to approve a sequel before directing STI founder Mark Cerny to start Sonic 2 in November 1991. Members of the original development team—including programmer Yuji Naka and designer Hirokazu Yasuhara—moved from Japan to California to join the project. Sonic 2 was intended to be faster and more ambitious than the first game. The development suffered setbacks, including language and cultural differences between the Japanese and American staff, and numerous levels were cut due to time constraints and quality concerns. As with the first game, Dreams Come True member Masato Nakamura composed the soundtrack.
Sonic 2 was widely anticipated, and Sega backed it with a $10 million marketing campaign. It was released in November 1992 to acclaim and received numerous year-end accolades. Critics considered Sonic 2 an improvement over the first game and praised the visuals, level design, gameplay, and music. The implementation of Tails and the multiplayer mode received criticism, while the special stages divided opinion. Sonic 2 broke video game sales records and grossed over $450 million in 1992. With six million copies sold worldwide, it is the second-bestselling Genesis game behind the original Sonic the Hedgehog.
Sonic 2 played a major role in keeping Sega competitive during the console wars of the fourth generation of video game consoles in the early 1990s. It continues to receive acclaim, often cited as one of the greatest video games of all time and one of the best Sonic games. Sonic the Hedgehog 3 and Sonic & Knuckles followed in 1994. Sonic 2 has been rereleased on various platforms via compilations and emulation, and a remake for iOS and Android devices, developed using the Retro Engine, was released in December 2013. A number of Sonic 2 prototypes have leaked since the game's release; the first, discovered in 1999, played a significant role in the development of the archaeogaming scene.