Water is fundamental to life and to the maintenance of an appropriate environment for physiological functions at the molecular, cellular, and organismal level. Water balance is also the principal mechanism of volume regulation in animals. The physical properties of water have profound effects on all biological structures and their function. Animal Osmoregulation has three main themes. The first deals with the physical properties of water, and its interactions with proteins, lipids, and biological membranes. Solutes affect the activity of water and thus the magnitude of the gradients driving water movement through osmosis. The distribution and transport of water in biological systems depends therefore on the properties of solutes, their distribution, and their transport. The second theme involves a detailed physical description of osmosis. This is followed by an explanation of the significance of osmotic regulation in animals inhabiting a wide variety of environments. Examples are explored for marine, freshwater, and terrestrial animals. A broad phylogenetic array of animals is discussed. Thirdly, the book deals with membranes as compartmental barriers. By definition, osmosis occurs through semi-permeable membranes. Membranes also, however, play a fundamental role in energy storage, energy transduction, solute transport, and sensory physiology. This volume approaches animal osmoregulation from the perspective of the physical laws that influence the structure of biological systems. It extends these concepts to explore the diversity of adaptations in the animal kingdom that deal with osmotic challenges in a variety of environments.