  The path integral approach offers not only an exact expression for the non-
equilibrium dynamics of dissipative quantum systems, but is also a convenient
starting point for perturbative treatments. An alternative way to explore the
influence of friction in the quantum realm is based on master equations which
require, however, in one or the other aspect approximations. Here it is
discussed under which conditions and limitations Markovian master equations can
be derived from exact path integrals thus providing a firm basis for their
applicability.
