  The results obtained on the particle mixing in Quantum Field Theory are
reviewed. The Quantum Field Theoretical formulation of fermion and boson mixed
fields is analyzed in detail and new oscillation formulas exhibiting
corrections with respect to the usual quantum mechanical ones are presented. It
is proved that the space for the mixed field states is unitary inequivalent to
the state space where the unmixed field operators are defined. The structure of
the currents and charges for the charged mixed fields is studied. Oscillation
formulas for neutral fields are also derived. Moreover the study some aspects
of three flavor neutrino mixing is presented, particular emphasis is given to
the related algebraic structures and their deformation in the presence of CP
violation. The non-perturbative vacuum structure associated with neutrino
mixing is shown to lead to a non-zero contribution to the value of the
cosmological constant. Finally, phenomenological aspects of the
non-perturbative effects are analyzed. The systems where this phenomena could
be detected are the $\eta-\eta'$ and $\phi-\omega$ mesons.
