  We present a detailed discussion of scalar wave propagation and light
intensity transport in three dimensional random dielectric media with optical
gain. The intrinsic length and time scales of such amplifying systems are
studied and comprehensively discussed as well as the threshold characteristics
of single- and two-particle propagators. Our semi-analytical theory is based on
a self-consistent Cooperon resummation, representing the repeated
self-interference, and incorporates as well optical gain and absorption,
modeled in a semi-analytical way by a finite imaginary part of the dielectric
function. Energy conservation in terms of a generalized Ward identity is taken
into account.
