  The multiple scattering of scalar waves in diffusive media is investigated by
means of the radiative transfer equation. This approach, which does not rely on
the diffusion approximation, becomes asymptotically exact in the regime of most
interest, where the scattering mean free path $\ell$ is much larger than the
wavelength $\lambda_0$. Quantitative predictions are derived in that regime,
concerning various observables pertaining to optically thick slabs, such as the
mean angle-resolved reflected and transmitted intensities, and the width of the
enhanced backscattering cone. Special emphasis is put on the dependence of
these quantities on the anisotropy of the cross-section of the individual
scatterers, and on the internal reflections due to the optical index mismatch
at the boundaries of the sample. The large index mismatch regime is studied
analytically, for arbitrary anisotropic scattering. The regime of very
anisotropic scattering, where the transport mean free path $\ell^*$ is much
larger than the scattering mean free path $\ell$, is then investigated in
detail. The relevant Schwarzschild-Milne equation is solved exactly in the
absence of internal reflections.
