The Transfer Pak is a device for the Nintendo 64 that allows data transfer between the N64 game and a Game Boy or Game Boy Color cartridge. The Transfer Pak has a Game Boy Color slot, and a part that fits into the expansion port of the Nintendo 64 controller. This was included with Pokémon Stadium, and was used to transfer the player's Pokémon teams from the Game Boy games (Red, Blue, and Yellow) to Pokémon Stadium. This was the first time that your favorite Pokémon could be seen in full 3-D. This trend continued, with Gold/Silver/Crystal being able to hook up to Pokémon Stadium 2.
It should be noted that of all the compatible games, only two of them will give benefits to the Game Boy or Game Boy Color game, but rather just the Nintendo 64 game. For example, while you can transfer your characters from the GBC version of Mario Tennis over to the Nintendo 64 version, you can't transfer the N64 version's characters over to the GBC version. The exception are Pokémon Stadium and Pokémon Stadium 2 which allow for the modification of the player's team. Pokémon Stadium also unlocked a minigame in Pokémon Yellow.