-->''[Shaun sits down and picks up a controller]''
-->'''{{Announcer|Chatter}}:''' Player One has entered the game!
-->'''Ed:''' Ain't you got work?
-->'''Announcer:''' Player Two has left the game!
-->''[Shaun puts down the controller and walks off]''
--->--''Film/ShaunOfTheDead''

In most video games, multiplayer is a matter of choosing the number of players at the start, and then getting a game going. This can be a problem if someone wants to join later. Ever have that situation where you're playing a game alone that you and your sibling both enjoy, then suddenly s/he walks into the room and wants to play?

The solution to that problem is Drop-In-Drop-Out Multiplayer. The ability to join at any time, and just as crucial, the ability to leave at any time (which helps you avoid that annoying problem where your younger brother suddenly doesn't want to play anymore, leaving with a second character just standing there). These titles have no real distinction between single and multiple-player games, since those initiated as one type can easily turn into the other and back.

This solves more than just the sibling problem – it's also is what makes games with large player limits practical. The GameLobby system, which requires players to get together ahead of time in a group and start the game all at once, would simply never allow for such games to happen by itself.

Since this means remaining playable with a fluctuating number of players, such games are usually either CoOpMultiplayer or heavily reliant on bots.
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!Examples:
[[AC:General]]
* Coin-op arcade machines (and many franchises originating from them like ''{{VideoGame/Gauntlet}}'') will traditionally have “PLAYER 2 INSERT COIN” flashing perpetually over the upper right corner of the screen where [=P2=]'s HUD goes, so that just dropping a bit of lucre into the corresponding slot will let any passerby join the game. This is especially useful with titles like the 6-player ''VideoGame/XMen'' arcade game. It's generally wise to ask players before joining in, out of courtesy.
** ''TimeCrisis 2'' and its successors allow you to lock out other players by selecting the Solo Play mode.
** ''VideoGame/{{Darius}} Burst Another Chronicle'' supports up to 4 players, and you can set a message at the beginning of the game to "request support" (that is, invite other players to join). You can also put the game into "Private Mode" to disallow new players.
* Ubiquitous in competitive online games, if for no other reason than to account for the possibility of some players experiencing connection problems in the early days. Now because, especially with MassivelyMultiplayer games having thousands of players in one level, and even many “normal” 100+ player games like ''VideoGame/{{Battlefield}}'' or ''[[VideoGame/EnemyTerritoryQuakeWars Enemy Territory]]'' , it would be almost impossible to get everyone together for a session in unison (though clan and LAN players try valiantly.)
* Games not particularly suited to this sometimes include aspects of it to deal with shortsighted players, especially games that tend toward lengthy matches. It's quite common, for instance, to see a larger ''BattleForWesnoth'' game where half the factions have been handed over to remaining players, the computer, or spectators.

[[AC:Other]]
* ''{{VideoGame/Diablo}}'' is almost MMORPG-like, with players able to join and leave at will, form into parties, and the game even applying DynamicDifficulty to compensate for the extra players.
* ''VideoGame/Left4Dead'' has four characters at all times, regardless of number of players. Joining the game means controlling one of them, leaving means switching them back to AI control.
* The ''{{Lego Adaptation Game}}s'' do this. They typically have a second character following the first player around, and any player who joins instantly takes control of the second character. Quitting the game means that character becomes AI-controlled again.
* The ''GuitarHero'' series introduced Party Play in ''Guitar Hero 5'', which allowed players to play as many songs as they like with the option of any player to pick any instrument and drop in when they like. When players join, the interface adjusts to fit more players on the screen, and when players leave, their note chart disappears.
* ''RockBand'' ''3'' introduced the ability for players to drop in/drop out at anytime, even during mid-song.
* Playing as Sonic and Tails in ''VideoGame/SonicTheHedgehog2'' or ''VideoGame/Sonic3AndKnuckles'', a player could grab the second game pad at any point to control Tails, while if there wasn't any input for about 30 seconds, the game will start controlling Tails again.
** Similarly for playing as Helper in the SNES version of ''VideoGame/KirbySuperStar'', except the game wouldn't take control of Helper until the second player's character died and was revived by the first.
* ''{{Sword of the Stars}}'' even allows players dropping out to leave the AI some basic instructions on how to run their empire and password lock their slot, in case they intend to come back and retake command later.
* ''VideoGame/MarioParty'' games allow you to set human players to computer players and vice versa at any time except during cutscenes and minigames.
* In ''MindJack'' if you allow your game to be open other players can "hack" into your single player game, to help or to hinder.
* Starting with ''VideoGame/NewSuperMarioBrosWii'', Nintendo's new retro games including ''VideoGame/KirbysEpicYarn'' and ''VideoGame/DonkeyKongCountryReturns'' take the approach of allowing players to join or leave only inbetween levels, but at least it's better than having to set the number of players ahead of time and have it be fixed. The WiiU sequel, ''VideoGame/NewSuperMarioBrosU'', allows players to join mid-game during a level, but not necessarily drop out mid-level.
* ''VideoGame/KirbysReturnToDreamland'' allows players to drop in and out during levels. They can simply press the + button to join, or push and hold the - button to drop out.
* ''VideoGame/TheWarriors'' allowed a 2nd player to control another Warrior character any time they jumped in and when they drop out, the character goes back to AI control.
* ''WorldInConflict'' is one of the few online RTS games that has this, made possible by the fact that it has no base building, so new players join at any time during a match without any gameplay disadvantage compared to players who've been there from the start. They ''do'' have the disadvantage of having less time to score points and the game also penalizes you for joining the already winning team if there are vacancies in the losing team (reducing the maximum amount of units you can field at once).
* ''[[VideoGame/{{Civilization}} Civilization 4]]'' had this, though it was a real pain coming into an AI civ where you didn't know where all your units were and had to immediately fortify them all.
* ''VideoGame/TeamFortress2'': allows players to come and go; some servers will even populate "empty" slots with AI bots so that even a handful of humans can still play a (somewhat) meaningful round. In addition, an autobalance and team scramble system is kept in place so that no one team is ever severely handicapped by mass quitting or one side repeatedly curb-stomping the other. Some servers disable one or both of these aforementioned systems, which plays hell with game balance as one side is left with only 4 or 5 players while the other has 12 or more, leading to a curbstomp battle with no one wanting to join the losing side.
* ''[[VideoGame/ItadakiStreet Fortune Street]]'' has the "Out To Lunch" option, where a player can take a break from playing and a computer AI will take over until the player opts back in. Notably, earning points allows you to purchase personality traits and roles to assign to your Mii that also determine the AI's general strategy. This is an unusual form of drop-in, drop-out in which existing players can leave and return, as opposed to new players joining.
* ''GearsOfWar'' didn't have this at all. ''2'' added it to campaign co-op (with a bot replacing Dom). ''3'' added it to campaign co-op (with bots replacing your other 3 party members) as well as versus multiplayer. It is also possible to join a Beast or Horde game mid-stream, but it requires joining a friend's game through the X-Box menu.
* The Franchise/TalesSeries usually enables this. Pause the game to set player 2 as controllable, then play. If you so choose, you can even make every character controlled by the AI.
* ''VideoGame/SinAndPunishmentStarSuccessor'', after a fashion. Since Player 2 is simply an on-screen cursor and cannot take damage, and there is no MultiplayerDifficultySpike, the second player can stop playing at any time without worrying about causing serious problems for the first player.
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