γγ Physics at Linear Colliders

A high-energy lepton-lepton collider will give us a unique possibility to study eγ and γγ interactions at high energies. The high-energy photons can be generated by Compton back-scattering of laser light on the high-energy lepton beams. With slightly reduced luminosities for eγ and γγ collisions compared to the lepton-lepton collider one, unique and complementary studies can be performed. A linear lepton-lepton collider also offers other photon sources that will be considered. We will focus on the complex properties of the photon itself and discuss some possibilities to gain new insight in its interactions at high energies and how to reveal the structure of it.