  We describe some salient features as well as some recent developments
concerning short-time deviations from the exponential decay law in the context
of Quantum Mechanics by using the Lee Hamiltonian approach and Quantum Field
Theory by using relativistic Lagrangians. In particular, the case in which two
decay channels are present is analyzed: the ratio of decay probability
densities, which is a constant equal to the ratio of decay widths in the
exponential limit, shows in general sizable fluctuations which persist also at
long times.
