  Two-frequency radiative transfer (2f-RT) theory is developed for classical
waves in random media. Depending on the ratio of the wavelength to the scale of
medium fluctuation 2f-RT equation is either a Boltzmann-like integral equation
with a complex-valued kernel or a Fokker-Planck-like differential equation with
complex-valued coefficients in the phase space. The 2f-RT equation is used to
estimate three physical parameters: the spatial spread, the coherence length
and the coherence bandwidth (Thouless frequency). A closed form solution is
given for the boundary layer behavior of geometrical radiative transfer and
shows highly nontrivial dependence of mutual coherence on the spatial
displacement and frequency difference. It is shown that the paraxial form of
2f-RT arises naturally in anisotropic media which fluctuate slowly in the
longitudinal direction.
