  The fermion mixing transformations are studied in the quantum field theory
framework. In particular neutrino mixing is considered and the Fock space of
definite flavor states is shown to be unitarily inequivalent to the Fock space
of definite mass states. The flavor oscillation formula is computed for two and
three flavors mixing and the oscillation amplitude is found to be momentum
dependent, a result which may be subject to experimental test. The flavor
vacuum state exhibits the structure of $SU(2)$ generalized coherent state.
