Physics Prospects at Future e+e- Colliders

I discuss the physics prospects at future high--energy $e^+ e^-$ linear colliders. After summarizing some aspects of these future machines, I will review a few topics in the Standard Model which can be studied at these colliders: top quark, $W/Z$ vector bosons and Higgs particles physics. A brief discussion on the exploration of extensions of the Standard Model such as Supersymmetry and Grand Unified Theories at $\ee$ colliders will also be given. (Invited talk given at Les Rencontres de Physique de la Vall\'ee d'Aoste, La Thuile March 3-9 1996).