  We review some of the main results of the quantum field theoretical approach
to neutrino mixing and oscillations. We show that the quantum field theoretical
framework, where flavor vacuum is defined, permits to give a precise definition
of flavor states as eigenstates of (non-conserved) lepton charges. We obtain
the exact oscillation formula which in the relativistic limit reproduces the
Pontecorvo oscillation formula and illustrate some of the contradictions
arising in the quantum mechanics approximation. We show that the gauge theory
structure underlies the neutrino mixing phenomenon and that there exist
entanglement between mixed neutrinos. The flavor vacuum is found to be an
entangled generalized coherent state of SU(2). We also discuss flavor energy
uncertainty relations, which imposes a lower bound on the precision of neutrino
energy measurements and we show that the flavor vacuum inescapably emerges in
certain classes of models with dynamical symmetry breaking.
