[en] Certain problems involving the decay of unstable states are examined within the context of nonrelativistic quantum mechanics. A model for the radiative decay of an excited atomic state which is similar to the Lee model is used to investigate the deviation of the actual decay law from a pure exponential law where the lifetime of the exponential is given by Fermi's Golden Rule. Both time-dependent and time-independent bounds are placed on the deviation and the dependence of these bounds on the properties of the transition current is examined. Both the case in which the modes of the radiation field are discrete and that in which the modes are continuous are considered. The effect of the presence of a second atom on the decay of an excited state is also studied