  The theory underlying neutrino oscillations has been described at length in
the literature. The neutrino state produced by a weak decay is usually
portrayed as a linear superposition of mass eigenstates with, variously, equal
energies or equal momenta. We point out that such a description is incomplete,
that in fact, the neutrino is entangled with the other particle or particles
emerging from the decay. We offer an analysis of oscillation phenomena
involving neutrinos (applying equally well to neutral mesons) that takes
entanglement into account. Thereby we present a theoretically sound proof of
the universal validity of the oscillation formulae ordinarily used. In so
doing, we show that the departures from exponential decay reported by the GSI
experiment cannot be attributed to neutrino mixing. Furthermore, we demonstrate
that the `Mossbauer' neutrino oscillation experiment proposed by Raghavan,
while technically challenging, is correctly and unambiguously describable by
means of the usual oscillation formalae.
