  We discuss the notion of quantum mechanical coherence in its connection with
time evolution and stationarity. The transition from coherence to decoherence
is examined in terms of an equation for the time dependence of the density
matrix. It is explained how the decoherence rate parameter arising in this
description is related to the ``unitarity defect'' in interactions with the
environment as well as the growth in entropy of the system. Applications to the
``Zeno-Watched Pot Effect'' and gravitational interactions are given. Finally,
some recent results on applications to macroscopic coherence with the rf SQUID,
where the transition from quantum to classical behavior could be studied
experimentally, are shown.
