A theory of polarization-dependent four-wave processes is developed fo
r photorefractive nonlinear media. It includes the classification of p
ossible elementary nonlinear processes, an analysis of their polarizat
ion and geometric properties, and a unified technique for the calculat
ion of wave amplitudes. The theory is applied to the description of di
fferent types of conical scattering in BaTiO3 and LiNbO3 crystals. Eff
ects of crystal anisotropy, angular and frequency detunings for intera
cting waves are taken into account. A good agreement between theory an
d experiment is demonstrated. Applications of the theory to treatment
of a number of important photorefractive effects are considered.