  After reviewing the description of an unstable state in the framework of Lee
Hamiltonians (valid both for Quantum Mechanics (QM) and Quantum Field Theory
(QFT)), we consider some theoretical aspects of non-exponential decays: the
case of two decay channels, the broadening of the energy spectrum at short
times, the effect of an imperfect measurement, the link to QFT, and the decay
of an unstable moving particle with definite momentum. All the presented
effects were not confirmed in experiments, hence are at the present stage
predictions.
