  We present a detailed, microscopic transport theory for light in strongly
scattering disordered systems whose constituent materials exhibit linear
absorption or gain. Starting from Maxwell's equations, we derive general
expressions for transport quantities such as energy transport velocity,
transport mean free path, diffusion coefficient, and absorption/gain length.
The approach is based on a fully vectorial treatment of the generalized kinetic
equation and utilizes an exact Ward identity (WI). While for loss- and gainless
media the WI reflects local energy conservation, the effects of absorption or
coherent gain are implemented exactly by novel, additional terms in the WI. As
a result of resonant (Mie) scattering from the individual scatterers, all
transport quantities acquire strong, frequency-dependent renormalizations,
which are, in addition, characteristically modified by absorption or gain. We
illustrate the influence of various experimentally accessible parameters on
these quanitities for dilute systems. The transport theory presented here may
set the stage for a theory of Random Lasing in three-dimensional disordered
media.
