  After reviewing the description of an unstable state in the framework of
nonrelativistic Quantum Mechanics (QM) and relativistic Quantum Field Theory
(QFT), we consider the effect of pulsed, ideal measurements repeated at equal
time intervals on the lifetime of an unstable system. In particular, we
investigate the case in which the `bare' survival probability is an exact
exponential (a very good approximation in both QM and QFT), but the measurement
apparatus can detect the decay products only in a certain energy range. We show
that the Quantum Zeno Effect can occur in this framework as well.
