  The precision and discovery potential of a neutrino factory based on muon
storage rings is summarized. For three-family neutrino oscillations, we analyze
how to measure or severely constraint the angle $\theta_{13}$, CP violation,
MSW effects and the sign of the atmospheric mass difference $\Delta m^2_{23}$.
The appearance of ``wrong-sign muons'' at three reference baselines is
considered: 732 km, 3500 km and 7332 km. We exploit the dependence of the
signal on the neutrino energy, and include as well realistic background
estimations and detection efficiencies. The optimal baseline turns out to be
$O$(3000 km).
