  Both Muon Colliders and Neutrino Factories require a muon source capable of
producing and capturing O(10^21) muons/year. This paper reviews the
similarities and differences between Neutrino Factory and Muon Collider
accelerator complexes, the ongoing R&D needed for a Muon Collider that goes
beyond Neutrino Factory R&D, and some thoughts about how a Neutrino Factory on
the CERN site might eventually be upgraded to a Muon Collider.
