The theory of the dielectric constant in solids is expounded. Peculiar features of electronic and ionic polarization of crystals, in particular, paraelectrics of displace type, are depicted. The physical nature of the thermal stability of microwave dielectrics, and different mechanisms of dielectric losses are analysed. The original method of microwave measurements of the parameters of ferroelectrics and the study of the films is proposed. Microwave properties of different type ferroelectrics, including relaxors, were studied. Microwave dielectric properties of the composites, especially air-dielectric composites with controllable effective permittivity are described.