This review gives a brief discussion of the structure of the Standard Model and its quantum corrections for testing the electroweak theory at current and future colliders. The predictions for the vector boson masses, neutrino scattering cross sections, and the Z resonance observables such as the width of the Z resonance, partial widths, effective neutral current coupling constants and mixing angles at the Z peak, are presented. Recent experimental data and their implications for the present status of the Standard Model are compared. Finally, the question of how virtual new physics can influence the predictions for the precision observables and the minimal supersymmetric standard model (MSSM)