Cheat codes are codes that can be entered into a video game to change the game's behavior. The practice of cheat codes and secrets in a video game was started in the Atari 2600 game Adventure. Afterwards codes were implemented and used by game developers to playtest certain aspects of their games; for example, a common use of a cheat code is to skip to a level in a game. In moddable games, such cheat modes are often left in released titles specifically for modder use.

The practice became widespread, and now many games have cheat codes intentionally included as a form of Easter egg or unlockable reward. Video game magazines have had sections devoted to providing these codes since the late 1980s, and occasionally codebooks are produced which provide cheat codes for a large number of games. There even used to be entire magazines dedicated to cheat codes for new games covering the top consoles of the day. Today, many codes, for games old and new alike, are found online at websites such as GameFAQs, GameWinners.com, or Cheat Planet. Another good source of cheat code information is Cheat Code Central.com, a site which contains cheat codes for nearly all console and PC games (even the, yet to be released to the US, Playstation 3).

The method of entering cheat codes varies; on video game consoles, which lack keyboards, the code is frequently a sequence of button presses. On computers such as the Amiga or PC, the code can be textual and entered using the keyboard, or more outlandish combinations of mouse, keyboard and controller may be required to activate the code. In modern games, the cheats are also often enabled through adding special settings in game configuration files.

While normal cheat codes are built into the game by the programmers, unofficial cheat codes can be created by manipulating the contents of memory address for a running game. On video game consoles this is done using a cheat cartridge. Users of some early home computers called these codes pokes, named after the command used to input them. Nowadays, cheating like this is considered "hacking" because the user must use external software or hardware to change values in the game memory.

Cheat codes are, by definition, considered cheating and most serious players do not use them. Sometimes, though, using cheat codes is necessary, particularly in case of bugs: If a serious game-stopping bug is encountered, a cheat code may be able to bypass it without the need to start the whole game over again from the beginning.

Sometimes cheat codes will actually spell a word when the buttons are pressed. For example, a code for the Sega Saturn game Bug! is "B, A, B, Y, Down, Right, A, L, Down." This reads "BABYSEALS", if you take down for "south", and right for "east." Another example is the classic "A, B, right, A, C, A, down, A, B, right, A" which spells Abracadabra.

Famous cheat codes

• Up, Up, Down, Down, Left, Right, Left, Right, B, A, Start — The Konami Code. First used in Gradius, more prominently used in Contra, has later been copied by many other games. It is also the name of a song by The Ataris.
• A, B, A, C, A, B, B – The blood code that allowed full blood and fatalities in the Sega Genesis version of Mortal Kombat.
• IDDQD, IDKFA and IDSPISPOPD from Doom by id Software.
• Up, Down, Left, Right, A+Start - The level select code for Sonic the Hedgehog.
• xyzzy – the magic word from Colossal Cave Adventure is often used as a cheat code.
• nethack -uwizard -D, which starts Nethack in "wizard mode." Plain -D command-line flag starts "discovery mode" which has fewer cheats.
• GOD - The cheat to activate god mode in the first 3 Commander Keen (and other ID Software) games.
• rosebud - Unlimited money on The Sims.
• thereisnospoon - Based on the quote from 1999 movie The Matrix, this code is used in many games to activate bullet-time.
• FLUFFYKIWIS entered in the Commodore Amiga version of The New Zealand Story.
• JUSTIN BAILEY - Entering this name as a password in the original NES Metroid with the bottom line all hyphens, started the player off in her swimsuit, and provided a great many powerups and items in the game.
• Down, R, Up, L, Y, B - Entering this in Street Fighter II as the Capcom logo appeared allowed both players to use the same character in Versus mode.
• DINGADINGDANGMYDANGALONGLINGLONG from the Commodore Amiga version of Sleepwalker. The rather hard-to-spell code is a lyric from the song Jesus built my Hot Rod by Ministry.
• It is a Good Day to Die from Warcraft II. This activated god mode. The code is a quote from the Star Trek series.de:Cheat