  We describe how the concepts of quantum open systems and the methods of
closed-time-path (CTP) effective action and influence functional (IF) can be
usefully applied to the analysis of statistical mechanical problems involving
quantum fields in gravitation and cosmology. In the first lecture we discuss in
general terms the relevance of open system concepts in the description of a
variety of physical processes, and outline the basics of the CTP and IF
formalisms. In the second lecture we illustrate the IF method with a model of
two interacting quantum fields, deriving the influence action via a
perturbative expansion involving the closed-time-path Green functions. We show
how noise of quantum fields can be defined and derive a general fluctuation-
dissipation relation for quantum fields. In the third lecture we discuss the
problem of backreaction in semiclassical gravity with the example of a scalar
field in a Bianchi Type-I universe. We show that the CTP effective action not
only yields a real and causal equation of motion with a dissipative term
depicting the effect of particle creation, as was found earlier, it also
contains a noise term measuring the fluctuations in particle number and
governing the metric fluctuations. The particle creation-backreaction problem
can be understood as a manifestation of a fluctuation-dissipation relation for
quantum fields in dynamic spacetimes, generalizing Sciama's observation for
black hole Hawking radiation. A more complete description of semiclassical
gravity is given by way of an Einstein-Langevin equation, the conventional
theory based on the expectation value of the energy momentum tensor being its
