Principles of photon colliders

Future linear colliders offer unique opportunities to study γγ, γe interactions. Using the laser backscattering method one can obtain γγ, γe colliding beams with energy and luminosity comparable to the electron-position luminosity or even higher. In this review physical principles of photon colliders are described and various problems, concerning the accelerator, laser, interaction region and luminosity are discussed. Some examples of physical processes are given.