General properties of radiation, propagation, and scattering of partially coherent light in linear and nonlinear media are investigated. In particular, universality of the functional form of the spectral degree of coherence of the wave field generated in a homogeneous medium by any two- or three-dimensional statistically stationary, isotropic source is demonstrated. The possibility of controlling the spectrum and the spectral degree of coherence of light by scattering from disordered anisotropic media is studied. A particular case of scattering from nematic liquid crystals is considered. A new class of spatial solitons with controllable coherence properties is presented, and a possible way of experimental realization of such solitons is discussed. The existence of a novel linear superposition principle for partially coherent spatial solitons with identical intensity profile is discovered, and the consequences of this principle are elucidated.