An expansion pack, expansion set, or supplement is an addition to an existing role-playing game, tabletop 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 expansion packs consist solely of additional content, most expansion packs require the original game in order to play. Games with many expansions often begin selling the original game with prior expansions, such as The Sims Deluxe Edition (The Sims with The Sims: Livin' Large). These bundles make the game more accessible to new players. When games reach the end of their lifespan, the publisher often releases a 'complete' or 'gold' collection which includes the game and all its subsequent expansions.

Some expansion packs do not require the original game in order to use the new content, as is the case with Blue Shift or Sonic & Knuckles. In some cases, a standalone expansion such as Heroes of Might and Magic III: The Shadow of Death or Dungeon Siege: Legends of Aranna includes the original game. Standalone expansions are preferred by game retailers since they require less shelf space and are generally easier to move since they don't carry the prerequisite of owning the original game. Often, enhanced games or anthologies released as "Game of the Year edition", "Director's Cut", etc. are examples of standalone expansions. In some standalone packs if you do not have the original or other you can't use certain units or races contained in the other games in multiplayer.

Expansion packs are most commonly released for computer games, but are becoming increasingly prevalent for video game consoles, particularly due to the popularity of online console services such as Xbox Live and the PlayStation Network. One such console expansion pack would be the Halo 2 Multiplayer Map Pack which contain 9 new maps and updates for the game that were only available for Xbox Live users to all gamers. The increasing number of multi-platform games has also led to the release of more expansion packs on consoles, especially stand-alone expansion packs (as described above). Command & Conquer 3: Kane's Wrath, for example, requires the original Command & Conquer 3: Tiberium Wars to play on the PC, but Xbox 360 versions of both the original Tiberium Wars and Kane's Wrath are available, neither of which require one another.

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 in order to play.

Sonic & Knuckles for the Sega Mega Drive/Genesis was unusual in that it functioned as both a stand alone cartridge and as an expansion pack for both Sonic the Hedgehog 2 and Sonic the Hedgehog 3.

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