  We review the physics potential of top mass measurements and the GigaZ/MegaW
options of the International Linear Collider (ILC) for probing New Physics
models and especially the Minimal Supersymmetric Standard Model (MSSM). We
demonstrate that the anticipated experimental accuracies at the ILC for the
top-quark mass, m_t, the W boson mass, M_W, and the effective leptonic weak
mixing angle, sin^2 theta_eff, will provide a high sensitivity to quantum
effects of New Physics. In particular, a new and more precise measurement of
sin^2 theta_eff, for which the experimental central value is currently obtained
from an average where the most precise single measurements differ by more than
three standard deviations, could lead to a situation where both the Standard
Model and the MSSM in its most general form are ruled out. Alternatively, the
precision measurements may resolve virtual effects of SUSY particles even in
scenarios where the SUSY particles are so heavy that they escape direct
detection at the LHC and the first phase of the ILC.
