Players control Sonic the Hedgehog or Knuckles the Echidna in their quests to save Angel Island; Sonic tries to stop Doctor Robotnik from re-launching his orbital weapon, the Death Egg, while Knuckles scuffles with Robotnik's minion, EggRobo.

Like previous Sonic games, players traverse side-scrolling levels at high speeds while collecting rings and defeating enemies.

Attaching the original Sonic the Hedgehog or any other Genesis game released prior to this will unlock the "Blue Sphere" minigame.

It received positive reviews; critics were impressed with the replay value and lock-on technology, despite its similarity to Sonic 3.

The levels also include cutscenes that differ based on the character selected, as Sonic and Knuckles are opponents for most of the game.

The first type has Sonic or Knuckles orbit floating, glowing spheres, jetting off each one when a button is pressed, while a fence of light approaches from the bottom and will remove the player from the stage if touched.

The second type involves bouncing around a room with a slot machine in its center with the intention of winning extra lives and power-ups.

If the player attaches the original Sonic the Hedgehog,[12] they are able to access all of the possible variations of the special stages, each with a unique level number and corresponding password.

[10] After the events of Sonic 3, Dr. Robotnik's orbital weapon, the Death Egg, crash-lands on Angel Island.

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.

He chases him to the damaged remains of Sky Sanctuary, where Mecha Sonic attacks Knuckles, but accidentally destroys EggRobo instead.

Sonic flies in piloting the biplane Tornado, and takes Knuckles and the Master Emerald back to Angel Island.

Sonic & Knuckles was developed in California by the Japanese members of Sega Technical Institute (STI).

They made Knuckles playable in Sonic 2 if that game was inserted because the team found his play style suited its levels.

They considered including the same functionality for the original Sonic the Hedgehog, but the developers felt Knuckles' abilities did not work with its level design.

[24] Blockbuster Video and MTV co-sponsored a tournament in which children were allowed to play Sonic & Knuckles.

The final tournament was held at Alcatraz Island in San Francisco, California, with a prize of $25,000 and the title "The World's Most Hardcore Gamer".

[52] Next Generation remarked, "Basically, this is the same Sonic game that Sega has sold for the last three years, just wrapped up better and with a prettier ribbon.

[47] Sega Magazine's review similarly praised the lock-on technology and the new innovation the unique cartridge offered, adding that Sonic & Knuckles' hidden stages and bosses would strongly add to the replay value of the combined game.

[45] Thomas praised the game for "impressive visuals that pushed the Genesis to its limits" and for the value added in the content unlocked with the lock-on technology.