Sonic & Knuckles is a 1994 platform video game in the Sonic the Hedgehog series for the Sega Mega Drive/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.

Sonic & Knuckles has been released on various Sonic and Sega themed compilations over the years, and digitally for the Wii's Virtual Console, and Xbox Live Arcade.

Main game

Because Sonic & Knuckles and Sonic the Hedgehog 3 were originally going to be one game, gameplay is nearly identical to Sonic 3. The game still plays as a 2D sidescrolling platformer, with the player directing Sonic through levels and around obstacles within a time limit of 10 minutes. However, while Tails is no longer a playable character, the game does introduce Knuckles the Echidna as a playable character.

Gameplay slightly differs between playing as Sonic or Knuckles. Sonic is able to run faster and jump higher than Knuckles, and can also utilize the special abilities of elemental shields. Meanwhile, Knuckles has the ability to glide downward through the air, climb up most walls, and break through certain barriers, occasionally allowing him to access areas Sonic cannot; Knuckles' limited jumping ability renders him unable to access certain areas that Sonic can. Knuckles' levels are noticeably more challenging than Sonic's, with certain bosses made more difficult.

The game has 2 bonus mini-games that can be accessed by hitting checkpoints with 20 or more rings and jumping through the stars. The first involves players attempting to earn rings in slot machines, where the other type sees the player traveling upwards through the use of magnetic orbs and flippers, collecting rings while avoiding a slowly rising forcefield that ends the mini-game.

The "special stages", the method in which Sonic collects the Chaos Emeralds in the game, play exactly like they do in Sonic 3, except with different layouts and the addition of yellow spheres which act as a 'spring', allowing the player to jump the distance of several red or blue spheres.

Lock-on technology

Sonic & Knuckles utilizes "lock-on technology" that lets the cartridge plus the Mega Drive/Genesis access data from Sonic the Hedgehog 2 and Sonic the Hedgehog 3 so that elements of both Sonic & Knuckles and the locked-on game are combined. To play these combined games, the hatch on top of the Sonic & Knuckles cartridge can be flipped open and the second game placed inside.

With Sonic the Hedgehog 3

Sonic the Hedgehog 3 and Sonic & Knuckles were originally to be released as one game, 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 Mega Drive/Genesis system, and then another game could be placed onto the top of the Sonic & Knuckles cartridge itself. 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.

With Sonic the Hedgehog 2

Referred to in-game as Knuckles the Echidna in Sonic the Hedgehog 2, the game is largely identical to Sonic the Hedgehog 2, with the exception that gameplay is altered due to Knuckles having separate abilities from Sonic or Tails. Knuckles can glide and climb walls, allowing him to gain access to areas that had been otherwise hidden or unreachable. Conversely, he cannot jump as high, making some situations, such as certain boss fights, especially the final one, more difficult. However, in this version of the game, the two player mode has been removed.

With Sonic the Hedgehog

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 all the available levels by inputting a twelve digit code, every number combination a different variation of a Special Stage level. This game is retroactively known as Blue Sphere in Sonic Mega Collection.

With any other game

If a Sega Mega Drive/Genesis game besides Sonic 2 or Sonic 3 is placed into the Sonic & Knuckles lock-on cartridge, the screen with Sonic, Tails, Knuckles and Robotnik stating "No Way" repeatedly is displayed with faded colors. However, by pressing the A, B and C buttons together, players can access a randomly generated Blue Sphere stage, based on the Special Stages of Sonic 3 and Sonic & Knuckles. Only one random stage can be accessed with this method per cartridge.