The book publicly outlines modern ideas about the properties and role of bound water as an important component of living and inanimate nature. The reader will get acquainted with the principles of spectroscopy and radio spectroscopy of bound water, with the laws used to interpret information about the bound state of water, with its types, origin and dynamics, with the world of hydrated substances - crystalline hydrates, zeolites, clay minerals, hydrated proteins and polysaccharides, with amazing transformations of bound water in human and animal tissues. The book is written in a popular language using little-known materials. Designed for a wide range of specialists - physicists, chemists, mineralogists, biologists, physiologists facing hydration problems or interested in this area.