This chapter considers the main mechanisms of the ocean microwave emission. Microwave emission characteristics are defined by three factors: dielectric properties of the seawater, geometrical, and volume nonuniformities of the air–sea interface. Dielectric permittivity is a function of electromagnetic wavelength, temperature, and salinity of water. There is strong dispersion of the complex permittivity in millimeter and centimeter ranges of electromagnetic wavelengths. At centimeter wavelengths, the influence of temperature on the permittivity and emissivity is more pronounced; at decimeter wavelengths, the influence of salinity on emissivity is much stronger. Microwave remote sensing is based on the measurements and analyses of the thermal emission of environmental media and/or natural objects. The effectiveness and reliability of microwave diagnostics of the ocean depend to a large extent on the knowledge of physical mechanisms and characteristics of thermal microwave emission. A relaxation theory of the dielectric constant of polar liquids was established in classical works.