The Physics Capabilities of mu^+ mu^- Colliders

We summarize the potential of muon colliders to probe fundamental physics. $W^{+}{W}^{-}$, \bar tt, and $Zh$ threshold measurements could determine masses to precisions $\mathrm{\Delta }{M}_W=6$ MeV, $\mathrm{\Delta }{m}_t=70$ MeV, and $\mathrm{\Delta }{m}_h=45$ MeV, to test electroweak radiative corrections. With $s$-channel Higgs production, unique to a muon collider, the Higgs mass could be pinpointed ($\mathrm{\Delta }{m}_h<1$ MeV) and its width measured. The other Higgs bosons of supersymmetry can be produced and studied by three methods. If instead the $WW$ sector turns out to be strongly interacting, a 4 TeV muon collider is ideally suited to its study.