  Quantum dynamics of a finite degrees of freedom is often much affected by
interaction with the larger environment of cosmic medium. In this lecture we
first review some recent developments of the theory of quantum dissipation in
the linear open system. In the second part we discuss two classes of
applications: decay of unstable particle in medium, and environmental effect on
the parametric particle production. The first subject of particle decay may
have important consequences on the scenario of baryogenesis due to a GUT heavy
particle. On the other hand, the parametric resonant particle production is
related to the reheating problem after inflation, and its environmental effect
is related also, perhaps more importantly, to the decay of moduli fields in
supergravity models.
