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.

10 pages,additional sections and one figure added