  We present a microscopic theory for multiple light scattering occurring in
inhomogeneous 3D media subject to an external magnetic field. Magneto-optical
effects (the Faraday effect and the Cotton-Mouton effect) occur inside the
small inhomogeneities. We thereby take into account the spatial anisotropy,
time-reversal-symmetry breaking and birefringence caused by the magnetic field,
and discuss the consequences for the diffusion tensor and the polarization
characteristics of the diffuse light. We will frequently compare our findings
to a similar phenomenon in dilute polyatomic gases: the Beenakker-Senftleben
effect. Coherent Backscattering and the field-field correlator $\left< E_i({\bf
B}) \bar{E}_j({\bf 0}) \right>$ are addressed, which have both been obtained
experimentally by the group of Maret {\em etal.}. All modifications in
transport theory due to the magnetic field exhibit a rather sensitive
dependence on the scattering phase shift of the individual scatterers.
