LU TP 99-35 hep-ph/9911444 November 1999 gamma-gamma Physics at Linear Colliders Christer Friberg A high-energy lepton-lepton collider will give us a unique possibility to study e-gamma and gamma-gamma 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 e-gamma and gamma-gamma 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. To appear in the Proceedings of the Workshop on the development of future linear electron-positron colliders for particle physics studies and for research using free electron lasers, Lund, Sweden, 23-26 September 1999.