Electroweak Gauge Theories and Alternative Theories at a Future Linear e+e- Collider

The measurement of Standard Model processes tests the validity of the model at a given scale and is simultaneously sensitive to new physics through loop effects or interference with the Standard Model amplitudes. A variety of studies has been done to see what a linear collider in the energy range MZ < sqrt{s} < 1 TeV can offer. The work that has been done within the ECFA/DESY workshop on linear colliders is reviewed, especially what was not included in the TESLA TDR.