e+e- linear colliders:: Theoretical basis and perspectives

The physics potential is summarized for e(+)e(-) linear colliders operating at center-of-mass energies up to roots = 1 TeV in the first phase, and at multi-TeV energies in the second phase. These machines will be ideal instruments to investigate the mechanisms for the breaking of the electroweak symmetries, the fundamental Higgs mechanism or dynamical alternatives. In supersymmetric theories, the spectrum of new particles and their interactions can be explored comprehensively and with high precision. The mechanism of supersymmetry breaking can be studied experimentally and the underlying fundamental theory can be reconstructed, eventually at scales near the Planck scale where particle physics and gravity are linked. The structure of space and time will be probed at distances down to 10(-18) cm in scattering experiments. Precision studies of the electroweak gauge bosons, the top quark and QCD supplement the physics program of these machines.