  We make an attempt to discuss in detail the effects originating from the
final state interaction in the processes involving production of unstable
elementary particles and their subsequent decay. Two complementary scenarios
are considered: the single resonance production and the production of two
resonances. We argue that part of the corrections due to the final state
interaction can be connected with the Coulomb phases of the involved charge
particles; the presence of the unstable particle in the problem makes the
Coulomb phase ``visible''. It is shown how corrections due to the final state
interaction disappear when one proceeds to the total cross-sections. We derive
one-loop non-factorizable radiative corrections to the lowest order matrix
element of both single and double resonance production. We discuss how the
infrared limit of the theories with the unstable particles is modified. In
conclusion we briefly discuss our results in the context of the forthcoming
experiments on the $W^+W^-$ and the $t\bar t$ production at LEP $2$ and NLC.
