Sonic Chaos (known as Sonic & Tails in Japan and Sonic the Hedgehog Chaos in Europe) is a platform game for the Sega Master System and Sega Game Gear, developed by Aspect and published by Sega in 1993. It was the last platform based Sonic game released for the Master System in Europe.
The Game Gear version was re-released in 2003 as a hidden game in Sonic Adventure DX for the Nintendo GameCube and PC and as an included game in Sonic Mega Collection Plus on the PlayStation 2, Xbox and PC. In 2009, the Master System version was released on the Virtual Console.
Sonic Chaos is notable for the fact that it is the first 8-bit Sonic title to feature Tails as a playable character, and also the first in the series to allow the player to manually control his flight. A sequel titled Sonic Triple Trouble (Sonic & Tails 2 in Japan) was released in 1994.
Source: Wikipedia, "Sonic Chaos", available under the CC-BY-SA License.