  We present a model of neutrino oscillations in the framework of quantum field
theory in which the propagating neutrino and the particles participating to the
production and detection processes are described by wave packets. The neutrino
state is a superposition of massive neutrino wave packets determined by the
production process, as naturally expected from causality. We show that the
energies and momenta of the massive neutrino components relevant for neutrino
oscillations are in general different from the average energies and momenta of
the propagating massive neutrino wave packets, because of the effects of the
detection process. Our results confirm the correctness of the standard
expression for the oscillation length of extremely relativistic neutrinos and
the existence of a coherence length.
