For the Nintendo 64 add-on, see Expansion Pak

An expansion pack is an addition to an existing game. These add-ons mostly add new game areas, weapons, objects, and/or an extended storyline to a complete and already released game. The original developer may contract out development of the expansion pack to third-party company, or it may choose to develop the expansion itself (see Hellfire for Diablo and Lord of Destruction for Diablo II). One of the first expansions (if not the first) was 1991's Wing Commander: The Secret Missions.

The price of an expansion pack is usually much less than that of the original game. As additions, most expansion packs require the original game in order to play. If a game has had older expansions, new expansion packs often feature all or some of the game's previous expansions. Games with many expansions sometimes begin selling the original game with the oldest expansions, such as The Sims Mega Deluxe (The Sims with The Sims: Livin' Large, The Sims: House Party, and The Sims: Hot Date). These collections make the game more accessible to new players, but publishers are careful to not make them compete with their own expansions that are still selling.

Some expansion packs do not require the original game in order to use the new content, as is the case with Blue Shift for Half-Life or Rainbow Six: Covert Operations Essentials for Rainbow Six. This type of expansion pack is often termed a "stand-alone expansion pack".

Expansion packs are most commonly released for computer games, but are becoming increasingly prevalent for video game consoles, especially with the popularity of online console services such as Xbox Live. One such console expansion pack would be the Halo 2 Multiplayer Map Pack which contain all 9 new maps and updates for the game that were only available for Xbox Live users to all gamers. More often than not, however, gaming companies choose to release new material for console games with updated re-releases that contain the complete original game with the new added features, such as Wipeout 3: Special Edition and Metal Gear Solid: Integral.

The term "expansion pack sequel" is sometimes used to describe a full game, most commonly a sequel that continues the storyline of the previous game, has completely new levels and characters, and adds a few new features, but has gameplay and graphics that are essentially identical (e.g. Doom II). The term is also sometimes used to describe a new game that's released after the original but before the official sequel. For example, Armored Core was followed by Armored Core: Project Phantasma and Armored Core: Master of Arena before Armored Core 2 was released. Also, the successors to Grand Theft Auto III (Grand Theft Auto: Vice City, Grand Theft Auto: San Andreas, and Grand Theft Auto: Liberty City Stories) are sometimes considered expansion pack sequels as they used only enhanced versions of the Grand Theft Auto III game engine, did not feature any significant gameplay changes, and were released before Grand Theft Auto IV.

Grand Theft Auto: London, 1969 was the first expansion pack released for the PlayStation. The game required the player to insert the London disc, remove it, insert the original Grand Theft Auto disc, remove it, then insert the London disc again.

• Episodic games
• List of gaming topics
• Mod (computer gaming)