  Feynman's path amplitude formulation of quantum mechanics is used to analyse
the production of charged leptons from charged current weak interaction
processes. For neutrino induced reactions the interference effects predicted
are usually called `neutrino oscillations'. Similar effects in the detection of
muons from pion decay are here termed `muon oscillations'. Processes considered
include pion decay (at rest and in flight), and muon decay and nuclear
$\beta$-decay at rest. In all cases studied, a neutrino oscillation phase
different from the conventionally used one is found. A concise critical review
is made of previous treatments of the quantum mechanics of neutrino and muon
oscillations.
