Dielectric, refractive and possibly magnetic properties of the media to be studied with microwave radiometry are important in several ways. First, the absorption coefficient follows from the imaginary part of the refractive index. The refractive index is a simple function of the dielectric constant and of the magnetic permeability. Second, the wave impedance and the scattering properties are related to the dielectric and magnetic properties. And finally, the ray path of the radiation depends on the refractive index in the propagating medium. Uncertainties of radiative transfer computations have often been limited by the insufficient accuracy of dielectric properties. The purpose of this chapter is to review the present knowledge in this field. A simplification to be made is the assumption that the magnetic permeability can be approximated by the value found in vacuum. It means that the small diamagnetic and paramagnetic effects can be ignored, and that ferromagneticity is not relevant to remote sensing. As an example water is slightly diamagnetic with a relative magnetic permeability of μ = 0.999991. In our approximation, this quantity is set to 1. The remaining electromagnetic key parameter is the complex dielectric constant. The dielectric constant of a substance depends both upon the electromagnetic field which interacts with the matter and upon the physical properties of the substance itself. For the fields considered in this book, e depends only upon the frequency, v, of the field. As for its dependence upon the matter, it will be a function of bulk properties such as temperature, pressure etc., a function of the physical constituents and of the molecular and atomic structure of the substance.