  Various precision determinations of the Fermi constant are compared. Included
are muon and (leptonic) tau decays as well as indirect prescriptions employing
$\alpha, m_Z, m_W, \ssthwmzms, \Gamma(Z\to\ell^+\ell^-)$, and $\Gamma(Z \to
\nu\bar \nu)$ as input. Their good agreement tests the standard model at the
$\pm 0.1%$ level and provides stringent constraints on new physics. That
utility is illustrated for: heavy neutrino mixing, 2 Higgs doublet models, S,
T, and U parameters and excited $W^{\ast^\pm}$ bosons (Kaluza-Klein
excitations). For the last of those examples, $m_{W^\ast}\gsims 2.9$ TeV is
found.
