  The physics responsible for neutrino masses and lepton mixing remains
unknown. More experimental data are needed to constrain and guide possible
generalizations of the standard model of particle physics, and reveal the
mechanism behind nonzero neutrino masses. Here, the physics associated with
searches for the violation of lepton-flavor conservation in charged-lepton
processes and the violation of lepton-number conservation in nuclear physics
processes is summarized. In the first part, several aspects of charged-lepton
flavor violation are discussed, especially its sensitivity to new particles and
interactions beyond the standard model of particle physics. The discussion
concentrates mostly on rare processes involving muons and electrons. In the
second part, the status of the conservation of total lepton number is
discussed. The discussion here concentrates on current and future probes of
this apparent law of Nature via searches for neutrinoless double beta decay,
which is also the most sensitive probe of the potential Majorana nature of
neutrinos.
