  Entropy generation in quantum sytems is tied to the existence of a
nonclassical environment (heat bath or other) with which the system interacts.
The continuous `measuring' of the open system by its environment induces
decoherence of its wave function and entropy increase. Examples of
nonrelativistic quantum Brownian motion and of interacting scalar fields
illustrate these general concepts. It is shown that the Hartree-Fock
approximation around the bare classical limit can lead to spurious semiquantum
chaos, which may affect the determination of entropy production and
thermalization also in other cases.
