We have searched for the electronic states that mediate oscillatory magnetic coupling in superlattices, and have found strong evidence that these are quantum well states, which are created by quantizing the momentum of s,p-band states perpendicular to the interfaces. In noble metal layers on ferromagnets we find such states at the Fermi level, where they are able to influence magnetic coupling and transport. They exhibit several traits connecting them with oscillatory magnetic coupling, such as periodicity and spin polarization.