  We review critically the main assumptions on which the standard theory of
neutrino oscillations is based. We show that all assumptions are realistic,
except the so-called "equal momentum assumption", which however is irrelevant.
We briefly review the covariant plane-wave derivation of neutrino oscillations
and a quantum field theoretical wave packet model of neutrino oscillations. We
show that both approaches lead to the standard expression for the oscillation
phase. The wave packet model allows also to describe the coherence of the
oscillations and the localization of the production and detection processes.
