Sonic & Knuckles

Since Sonic & Knuckles and Sonic the Hedgehog 3 were initially developed as one game, their gameplay is similar: both are 2D side-scrolling platformers with similar level design, graphics, and game mechanics. However, in Sonic & Knuckles, unlike in Sonic 3, the player chooses either Sonic or Knuckles at the title screen, Miles "Tails" Prower is not available to select, and the player cannot control two characters together. There is also no multiplayer mode or save feature.

The player character moves through six levels, each divided into two acts. The first act of each level ends with a miniboss fight with one of Dr. Robotnik's robots, while the second ends with a regular boss fight with Robotnik (or EggRobo in Knuckles's campaign). Sonic and Knuckles traverse levels differently; Sonic can jump higher and has access to unique shield abilities, whereas Knuckles can glide and climb most walls. The levels also include cutscenes that differ based on the character selected, as Sonic and Knuckles are opponents for most of the game.

After the events of Sonic 3, Dr. Robotnik's orbital weapon, the Death Egg, crash-lands on Angel Island. Sonic travels to Angel Island to retrieve the Chaos Emeralds to defeat Robotnik, and once again comes into conflict with Knuckles, who believes Sonic is trying to steal the Emeralds for himself. In Hidden Palace Zone, Sonic fights and defeats Knuckles, only to discover Robotnik stealing the Master Emerald, the secret to the island's levitation powers.

Knuckles attacks Robotnik, but Robotnik shocks him with electricity. Knuckles shows Sonic a portal that leads them to Sky Sanctuary, where the Death Egg is relaunching. Sonic infiltrates the Death Egg and defeats Robotnik as Super Sonic, retrieving the Master Emerald and returning it to Angel Island as it rises back into the sky.

Sonic & Knuckles was developed in California by the Japanese members of Sega Technical Institute (STI). Sonic 3 and Sonic & Knuckles were originally planned as a single game; according to designer Takashi Iizuka, the team "really wanted to hit a home run", with larger stages that took longer to develop. As ROM cartridges were too small to contain the game and Sega wanted it released in time for a promotional deal with McDonald's in the US, Sonic 3 was split in two, with Sonic & Knuckles as the second part. As Mushroom Hill became the first stage of Sonic & Knuckles, the developers lowered its difficulty.

Despite the split, the developers wanted the games to feel like a single experience. According to Roger Hector, vice president and general manager of STI, the lock-on technology was conceived between the releases of Sonic the Hedgehog (1991) and Sonic the Hedgehog 2 (1992).

The Genesis version sold at least 1.24 million copies in the United States. Sonic 3 and Sonic & Knuckles sold a combined 4 million cartridges worldwide.

Critics praised Sonic & Knuckles, despite its similarity to its predecessor. The four reviewers of Electronic Gaming Monthly named it their "Game of the Month". They lauded the lock-on technology and remarked that despite that being "more of the same, it still is an exceptional game". GamePro commented that the ability to play as Knuckles makes it essentially two games on a single cartridge, the game is more challenging than Sonic 3, and the ability to hook the cartridge up to Sonic 2 and 3 makes those games "worth playing again".

For the series' twentieth anniversary in 2011, Sega released Sonic Generations, a game that remade aspects of various past games from the franchise. A remake of the Sky Sanctuary stage was made for the Xbox 360, PlayStation 3, and PC versions of the game, while the Mushroom Hill level was remade for the Nintendo 3DS version of the game. Sonic & Sega All-Stars Racing features a track inspired by the Death Egg as downloadable content along with the Metal Sonic character. The sequel, Sonic & All-Stars Racing Transformed, featured the same race course, and an additional new one inspired by the Sky Sanctuary stage.

Re-imagined versions of the Flying Battery and Lava Reef stages, along with the Blue Sphere bonus game, appear in the 2017 game Sonic Mania. The 2022 Sonic the Hedgehog 2 movie draws inspiration from Sonic & Knuckles, though it is not a direct adaptation.