  We first review the usefulness of the Wigner distribution functions (WDF),
associated with Lindblad and pre-master equations, for analyzing a host of
problems in Quantum Optics where dissipation plays a major role, an arena where
weak coupling and long-time approximations are valid. However, we also show
their limitations for the discussion of decoherence, which is generally a
short-time phenomenon with decay rates typically much smaller than typical
dissipative decay rates. We discuss two approaches to the problem both of which
use a quantum Langevin equation (QLE) as a starting-point: (a) use of a reduced
WDF but in the context of an exact master equation (b) use of a WDF for the
complete system corresponding to entanglement at all times.
