  After a review of the problems associated with the conventional treatment of
particle oscillations, an oscillation formula is derived within the framework
of quantum field theory. The oscillating particle is represented by its
propagator and the initial and final states by wave packets. It is obviously
relativistic from the start and moreover applies both to stable (neutrinos) and
unstable particles (K and B mesons, unstable neutrinos). CPLEAR and DAFNE
experiments are studied as examples, with special attention directed to CP
violation. The problems resulting from equal energies/momentum/velocities
prescriptions are analyzed and solved. Oscillations of associated particles are
found to be nonexistent. The relativistic generalization of the
Wigner-Weisskopf equation is also derived.
