Studies of critical opalescence near phase transitions in different systems are reviewed. The fundamentals of the modern approach to the description of the propagation and scattering of light in media with large nonuniformities are presented. The experimental data on the coefficient of extinction and integrated intensity of the scattered light are analyzed and different models for the correlation function are discussed. Different methods for studying multiple scattering of light and methods for eliminating it from the measured intensity are examined. The kinetic properties of systems in the critical region are examined. Attention is devoted primarily to the experimental data obtained by the methods of correlation spectroscopy and to the comparison of these data with the predictions of different theoretical models. Significant attention is devoted to the study of phase transitions in liquids by the methods of light scattering as well as the problem of propagation and scattering of light in the nematic phase, where fluctuations of the director exhibit the same behavior as at the critical point. In conclusion, phase transformations in micellar solutions, to which a great deal of attention has been devoted in recent years, are studied.