Water is the most important substance in our everyday life, and has been studied as no other medium. While it has a relatively simple atomic composition, water nevertheless presents an astonishing variety of manifestations of its interaction with electromagnetic waves of the different wavelengths from radio frequencies to X-rays, representing its uniqueness compared to other dielectrics. In this chapter, the response of water to external electromagnetic radiation is considered in an extended frequency range from 0 to 10 15 Hz. The independent view on the separate parts of the spectrum, such as the dielectric constant, DC conductivity, radio wave, microwave, terahertz, and IR absorption, is provided along with a comprehensive view on the entire spectrum as a whole by means of sum rules, Kramers–Kronig analysis and the isotope effect. A curious reader will find a complete description of water’s electrodynamic parameters, as well as specific questions regarding the dynamic structure of water.