  Linear colliders (LC) on the energy 0.5-1 TeV are considered as the next step
in the particle physics. High acceleration gradients, small beam sizes,
precision tolerances, beam collision effects are main problems for linear
colliders. In this paper we discuss physics motivation, parameters and status
of current LC projects, e+e-, gamma-gamma and gamma-electron modes of
operation, physical limitations on the energy and luminosity. Present
technologies allow to reach energies about 5 TeV with adequate luminosities.
Advanced technique based on plasma and laser method of acceleration can provide
much higher accelerating gradients, however, perspectives of these methods for
high energy colliders are still under big question. Linear colliders with
energies above 10 TeV are hard for any acceleration technology. Speculations on
possibility of PeV linear colliders based on ponderomotive laser acceleration
are just not serious and contain several mistakes on conceptual level. It is
shown that due to radiation in the transverse laser field, methods of
acceleration based on laser bunch ``pressure'' do not work at high energies.
