The Game Genie is line of cheat devices that was manufactured by Galoob. These devices can be connected to a game cartridge, and then inserted into a console, functioning essentially as physical extension of the cartridge. By using codes input into a Game Genie menu before booting the game proper, hex values in the game could be altered to add abilities or access content that game developers did not intend.
Due to their nature as unsupported ROM hack devices, Game Genie codes are not guaranteed to work. And because some games are released in multiple revisions with slight changes, a Game Genie code that may work in the original printing may not work in a later version.
Game Genies were released for the NES, SNES, Game Boy, Genesis, and the Game Gear. The Game Genie was the predecessor to devices such as the Action Replay, Code Breaker, and GameShark, which provided similar functionality for more modern systems such as the Nintendo 64, PlayStation 2, and Xbox.
Hyperkin later acquired the Game Genie name and has since released a save editor compatible with the PlayStation 3 and a cheat device for the Nintendo DS. However, these products should not be confused in any way with the original line of Game Genie devices.
The Game Genie for the NES had a unique quirk -- since the games had to be pushed in and down into the console, the Game Genie had to be built with a system to override that feature, which made it so the Nintendo game would be jutting out the front of the console. This design put stress on the contact pins inside the NES and could lead to the pins becoming bent and twisted. The damage could potentially lead to the NES being unable to play games without the use of a Game Genie inserted.
On the SNES, the Game Genie worked well until the release of Nintendo's Super FX Chip. Games featuring these chips had extra chips that plugged into the left and right sides of the main slot, though at the time of the release of the Game Genie, these slots were unused so there was no need to create chips to connect to them. Accordingly, the Game Genie was unable to access all the code necessary to add the cheats. There were some games that did work even without the extra plugs -- Mega Man X2 and Mega Man X3 both worked perfectly with the Game Genie.
The Game Boy version of the Game Genie was a cartridge similar to other Game Boy cartridges, except it had another slot on top to insert a regular Game Boy cartridge. This version of the Game Genie was also packaged with a small code booklet. When used in the Game Boy Color, it had to be pushed down. This problem can be fixed by Using it in a GBA SP.
The Genesis/Mega Drive version of the Game Genie can also serve as a country converter. Most Genesis cartridges were only region-locked by the shape of the cartridge and a few bytes in the header of the ROM, and by using the Game Genie, the lock could be circumvented.
The Game Gear version of the Game Genie had quite an interesting design. Upon being pushed into the cartridge slot, another slot would open on the top of the Game Genie where players could place the regular Game Gear cartridge. This version of the Game Genie also came with a small code booklet.