  The potential of present and anticipated future electroweak precision data,
including the Higgs boson and top quark masses, for testing quantum effects of
the electroweak theory is investigated in the context of the Minimal
Supersymmetric Standard Model (MSSM). The present status of the theoretical
predictions is analyzed. The impact of the parametric uncertainties from the
experimental errors of the input parameters is studied, and an estimate for the
remaining uncertainties from unknown higher-order corrections is given both in
the Standard Model (SM) and the MSSM. Examples of electroweak precision tests
in the mSUGRA scenario and the unconstrained MSSM are analyzed, and the status
of the global fit to all data is discussed.
