The problem of particle oscillation is considered in a pedagogical and
comprehensive way. Examples from K, B and neutrino physics are given.
Conceptual difficulties of the traditional approach to particle oscillation are
discussed. It is shown how the probability current density and the wave packet
treatments of particle oscillations resolve some problems. It is also shown
that only full field theoretical approach is free from conceptual difficulties.
The possibility of oscillation of particles produced together with kaons or
neutrinos is considered in full wave packet quantum mechanics language. Precise
definition of the oscillation of particles which recoil against mixed states is
given. The general amplitude which describes the oscillation of two particles
in the final states is found. Using this EPR-type amplitude the problem of
oscillation of particles recoiling against kaons or neutrinos is resolved. The
relativistic EPR correlations on distances of the order of coherence lengths
are considered.Comment: revtex, epsfig, 4 figures, 28 pages. Presented at the XXXVIII Cracow
School of Theoretical Physics, Zakopane, June 1-10, 1998, to appear in Acta
Phys. Po
