We describe the physics potential of e + e − linear colliders in this report. These machines are planned to operate in the first phase at a center-of-mass energy of 500 GeV, before being scaled up to about 1 TeV. In the second phase of the operation, a final energy of about 2 TeV is expected. The machines will allow us to perform precision tests of the heavy particles in the Standard Model, the top quark and the electroweak bosons. They are ideal facilities for exploring the properties of Higgs particles, in particular in the intermediate mass range. New vector bosons and novel matter particles in extended gauge theories can be searched for and studied thoroughly. The machines provide unique opportunities for the discovery of particles in supersymmetric extensions of the Standard Model, the spectrum of Higgs particles, the supersymmetric partners of the electroweak gauge and Higgs bosons, and of the matter particles. High precision analyses of their properties and interactions will allow for extrapolations to energy scales close to the Planck scale where gravity becomes significant. In alternative scenarios, like compositeness models, novel matter particles and interactions can be discovered and investigated in the energy range above the existing colliders up to the TeV scale. Whatever scenario is realized in Nature, the discovery potential of e + e − linear colliders and the high-precision with which the properties of particles and their interactions can be analysed, define an exciting physics programme complementary to hadron machines.

We investigate the Higgs sector of a nonlinear supersymmetric standard model at LEP 1 and LEP 2, as well as at future linear e + e − colliders with √ s = 500, 1000, and 2000 GeV. The LEP 1 data do not put any constraints on the parameters of the model, and allow a massless Higgs boson in particular. For LEP 2, there are remarkable differences between the Higgs productions at √ s = 175 GeV on the one hand and that at √ s = 192 GeV and 205 GeV on the other hand. The case for √ s = 175 GeV is similar to LEP 1, whereas those for √ s = 192 GeV and 205 GeV will be able to give experimental constraints on the parameters. Finally the e + e − colliders with √ s = 500, 1000, and 2000 GeV are most probably able to test the model conclusively.

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