Introduction and recent developments in gamma-gamma, gamma-electron   colliders
  High energy photon colliders (gamma-gamma, gamma-electron) based on backward Compton scattering of laser light is a very natural addition to e+e- linear colliders. In this report we consider mainly this option for the TESLA project. Recent study has shown that the horizontal emittance in the TESLA damping ring can be further decreased by a factor of four. In this case the gamma-gamma luminosity luminosity in the high energy part of spectrum can reach 0.3--0.5 L_e+e-. Typical cross sections of interesting processes in gamma-gamma collisions are higher than those in e+e- collisions by about one order of magnitude, so the number of events in gamma-gamma collisions will be more that in e+e- collisions. The key new element in photon colliders is a very powerful laser system. The most straightforward solution is ``an optical storage ring (optical trap)'' with diode pumped laser injector which is today technically feasible. This paper briefly review the status of a photon collider based at TESLA, its possible parameters. 
Valery Telnov (Institute of Nuclear Physics, Novosibirsk, Russia)