The Transfer Pak connects Pokémon Stadium on Nintendo 64 to Game Boy Pokémon cartridges.
The accessory fits into the back of a Nintendo 64 controller and accepts compatible Game Boy cartridges. In Pokémon Stadium, it allows Pokémon from Red, Blue, and Yellow to be transferred into the Nintendo 64 game for viewing, organizing, and battling.
This connection was important because Pokémon Stadium focused on 3D battles while the original Pokémon adventures had been played on the Game Boy. The Transfer Pak linked the portable collection system with Stadium’s larger-screen battles and additional features.
It is easy to confuse the Transfer Pak with the Game Link Cable, which connected Game Boy systems to one another. A Controller Pak stored Nintendo 64 save data, but it did not connect the console to Pokémon cartridges.