  Using the laser backscattering method at future linear colliders one can
obtain gamma-gamma and gamma-electron colliding beams (photon colliders) with
energy and luminosity comparable to that in e^+e^- collisions. This option has
been included in the pre-conceptual designs of linear colliders and in work on
a Technical Design Report which is in progress. The physics motivation for
photon colliders is quite clear. The proof of its technical feasibility and the
search for the best solutions is of first priority now. A key element of a
photon collider is a laser with high peak power and repetition rate. One very
promising way to overcome this problem is the optical cavity approach which is
discussed in this paper. A very high gamma-gamma luminosity could be achieved
by further decreasing the beam emittances. This will be very challenging. One
possible way is laser cooling of electron beams. This method is discussed in my
second talk at this symposium. The solution to the first problem is vital for
photon colliders and provides an interesting physics program. Solution of the
second problem makes photon colliders a very powerful instrument for study of
matter, the best for study of many phenomena. How to achieve these goals is the
subject of this talk.
