The water content is of considerable significance for the physical and chemical properties of many materials. The correct knowledge of the water content is therefore important in many areas of human activities, particularly in agriculture and food production, as well as in numerous branches of industry, Largely independent of special features of a sample, electromagnetic wave interactions with the dipolar water enable the non-destructive and, in most cases, non-invasive determination of the water content. In order to reach an adequate accuracy in the measurements the probes have to be carefully designed, matched to the systems under study. For this reason knowledge of the dielectric spectra of the aqueous phase is a prerequisite. In this article examples of contributions to the spectra are presented in order to show effects originating from relevant parameters like frequency, temperature, and pressure, and also from various solutes.
