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, Cheat Codes Galore and GameWinners.

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 only use them for experimentation, if at all. 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 down is read "south", and right is read "east." Another example is the classic "A, B, right, A, C, A, down, A, B, right, A" which spells Abracadabra.

Famous cheat codes

• Noclip - Often used in many First Person Shooters. This cheat will generally allow a player to fly and not be able to have contact or interact with walls, floors, objects (including powerups or weapons), and other players, passing through them as if they were a ghost. Players still retain the ability to use whatever weapons or equipment they originally had.
• Up, Up, Down, Down, Left, Right, Left, Right, B, A — 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 allows full blood and fatalities in the Sega Genesis version of Mortal Kombat.
• M, I and L together in the original Wolfenstein 3D gives full health, full ammo, all weapons and both keys. Also reduces the score to zero.
• IDDQD, IDKFA, IDCLIP and IDSPISPOPD from Doom by id Software.
• Impulse 101; the code for all weapons, all ammunition, all shield strength in the popular Half-Life series and its mods.
• Up, Down, Left, Right, A+Start - The level select code for Sonic the Hedgehog.
• ZELDA - By entering this as their character name in the original The Legend of Zelda, the player skips directly to the game's second quest without playing the first one.
• 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 three Commander Keen (and other id Software) games. It also works in most Doom-based games, including Sonic Robo Blast 2.
• rosebud followed by a very large number of semi-colons (;) and input multiple times - 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. In Deus Ex this code will enable "May Tricks mode"(digital rain textures.)
• FLUFFYKIWIS entered in the Commodore Amiga version of The New Zealand Story.
• JUSTIN BAILEY - Entering this code as a password in the original NES Metroid with the bottom line all hyphens, starts Samus Aran off in her swimsuit, and provided a great many powerups and items in the game. (Ironically, this was not a 'true' cheat code implemented by the developers; rather, it was discovered by accident.)
• Down, R, Up, L, Y, B - Entering this in the SNES version of Street Fighter II as the Capcom logo appears allows 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 activates god mode. The code is a quote from the Star Trek series.
• Ctrl+Alt+X - This key combination would bring up a cheat window in certain Maxis games such as Streets of SimCity, Simcopter, etc... The Sims uses Ctrl+Shift+C, while Simcity 3000 uses Ctrl+Alt+Shift+C.
• 19, 65, 09, 17 at the sound test is the level select code for Sonic the Hedgehog 2. It's also the birthdate of Yuji Naka.
• There is no cow level - from Starcraft. This cheat code recurs in many Blizzard Entertainment games. This refers to an internet rumor that stated Diablo (another game by Blizzard) had a secret level where all enemies would be cows. Diablo II really did have this secret.
• Ctrl-Alt-X - Skips the Trivia Quiz Age Check in the original Leisure Suit Larry.
• moveobjects_on - In The Sims and all following sequels and expansion packs, moveobjects_on allows for all objects to be moved, from cars to mailboxes to Sims.
• Ctrl+Alt+End - This key combination would make you skip a level in The Dark Project. It is considered as a cheatcode with a nasty habit. This due to the fact that most people accidentally would press Ctrl+Alt+Del which would bring up the windows task manager.

Computer and video game gameplay

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