Sonic & Knuckles (ソニック&ナックルズ Sonikku to Nakkuruzu?) is a 1994 platform video game in the Sonic the Hedgehog series for the Sega Genesis. It was developed in the United States at Sega Technical Institute by members of Sonic Team, and was published by Sega worldwide on October 18, 1994.In addition to being a standalone game, through a system dubbed "lock-on technology," Sonic & Knuckles acts as an expansion pack to both Sonic the Hedgehog 2 and Sonic the Hedgehog 3, the latter with which it was developed in tandem.

"Blue Sphere" redirects here. For the video game in the Star Ocean series, see Star Ocean: Blue Sphere.If the player uses the original Sonic the Hedgehog or Sonic Compilation, a screen with Sonic, Tails, Knuckles and Robotnik stating "No Way" repeatedly is displayed. By pressing the A, B and C buttons together, players can access a minigame based on the Special Stages of Sonic 3 and Sonic & Knuckles. Players can enter a twelve digit code on this screen, with every number combination generating a different variation of a Special Stage level. This game is retroactively known as Blue Sphere in Sonic Mega Collection.

Sonic the Hedgehog 3 and Sonic & Knuckles were originally to be released as one game,[4] but due to time and financial constraints, it was later split into two separate games. However, Sonic & Knuckles contained "Lock-on Technology", where the Sonic & Knuckles cartridge would be put in the Sega Genesis system, and then another game could be placed onto the top of the Sonic & Knuckles cartridge itself.[4] When Sonic 3 is used in this manner, it enables the player to play through both games as one, titled in-game as Sonic 3 & Knuckles. This features several additions to the games not available otherwise, such as small alterations to the levels, and being able to play through Sonic 3 levels as Knuckles, or Sonic & Knuckles levels as Tails, and the ability to save progress in all of the game's levels.Additionally, this was the only way to collect Super Emeralds, which are earned by accessing Special Stages in the Sonic & Knuckles levels after collecting all seven Chaos Emeralds from Sonic 3. When all Super Emeralds have been collected, Sonic, Knuckles and Tails can transform into Hyper Sonic, Hyper Knuckles and Super Tails respectively, each with unique abilities.