  The high-energy lepton collider TESLA can be operated in the GigaZ mode on
the Z resonance, producing a billion Z bosons per year. This will allow the
measurement of the effective electroweak mixing angle to an accuracy much
better than one part in 10,000. Similarly, the W boson mass is expected to be
measurable with an error of 6 MeV near threshold. We discuss the impact of
these observables on the accuracy with which the Higgs boson mass can be
determined from loop corrections within the Standard Model. We also study
indirect constraints on new mass scales within the Minimal Supersymmetric
Standard Model.
