  We present an expression for the transition probability between Dirac or
Majorana neutrino flavors obtained from first principles within quantum field
theory. Our derivation is based on a standard quantum mechanical setup and
includes the specific mechanism of neutrino production only in as much as it
specifies the initial state. Our expression for the transition probability
reproduces the usual formula in the plane wave limit and shows the correct
non-relativistic and ultra-relativistic behaviors. It also allows a simple
understanding of the decoherence of the oscillations and of the question of the
arrival times of the different neutrino mass eigenstates. We show numerical
examples for the case of two neutrino generations.
