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The Secret Missions for Wing Commander was one of the first video game expansion packs.

An expansion pack is an addition to an existing pen-and-paper RPG, game or video game. These add-ons usually add new game areas, weapons, objects, and/or an extended storyline to a complete and already released game. While board game expansions are typically designed by the original creator, video game developers sometimes contract out development of the expansion pack to third-party company, or it may choose to develop the expansion itself (see Hellfire for Diablo). Board games and RPGs have been marketing expansions since at least the early 1970s.

The price of an expansion pack is usually much less than that of the original game. As they consist of game additions, most expansion packs require the original game in order to play. In video games, if a game has had multiple 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, the Nitro Pack for Interstate '76, Company of Heroes: Opposing Fronts for Company of Heroes, or Rainbow Six: Covert Operations Essentials for Rainbow Six. This type of expansion pack is often termed a "stand-alone expansion pack". While found in some video games this is much rarer in RPGs and board games, which prefer to use "spin-off" titles to capitalize on an original game's popularity (see Vampire: The Masquerade and Vampire: The Dark Ages).

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" 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. The successors to Grand Theft Auto III (Grand Theft Auto: Vice City, Grand Theft Auto: San Andreas, Grand Theft Auto: Liberty City Stories and Grand Theft Auto: Vice 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.

Consumers who don't check carefully may confuse an expansion pack with a stand-alone game. They may buy it thinking they've found a bargain-priced game, only to find out that they've actually bought an expansion pack that's useless without the original game. This is especially common among people not used to playing games, who may be buying it as a gift for someone else. This phenomenon was parodied in an episode of Mission Hill, "Happy Birthday, Kevin".

• Data disk (computer gaming)
• Episodic games
• Mod (computer gaming)
• Gold edition