  An observation of Charged Lepton Flavor Violation (CLFV) would be unambiguous
evidence for physics beyond the Standard Model. The Mu2e and COMET experiments,
under construction, are designed to push the sensitivity to CLFV in the mu to e
conversion process to unprecedented levels. Whether conversion is observed or
not, there is a strong case to be made for further improving sensitivity, or
for examining the process on additional target materials. Mu2e-II is a proposed
upgrade to Mu2e, with at least an additional order of magnitude in sensitivity
to the conversion rate over Mu2e. The approach and challenges for this proposal
are summarized. Mu2e-II may be regarded as the next logical step in a continued
high-intensity muon program at FNAL.
