Which accessory linked Pokémon Stadium to Game Boy Pokémon cartridges?

The Transfer Pak linked Pokémon Stadium to Game Boy Pokémon cartridges.

Pokémon Stadium was a Nintendo 64 game that let players use Pokémon from Pokémon Red, Blue, and Yellow. The Transfer Pak plugged into the Nintendo 64 controller, and a compatible Game Boy cartridge was inserted into the accessory.

This connection allowed players to view, organize, store, trade, and battle with Pokémon from their handheld games. It also made it possible to use a trained party on the Nintendo 64's three-dimensional battlefields.

The Transfer Pak was not the same as the Rumble Pak, which provided vibration feedback, or the Controller Pak, which stored certain Nintendo 64 save data. Pokémon Stadium's compatibility helped connect the handheld and home-console sides of the franchise.