The book provides a systematic and complete presentation of the current state of the phenomenological theory of Landau phase transitions in relation to various phase transitions in crystals. The apparatus of the theory of representations of spatial groups and methods for determining the order parameter for a given phase transition are described in detail. Symmetric theory of phase domains, the current state of incommensurable phases, symmetric aspects of fluctuation theory. are considered. For researchers - theorists and experimenters engaged in the field of solid state physics, as well as graduate students and senior students of physical and physical-technical specialties of universities .