Presenting an analysis of the water relationships of the major groups of organisms: fungi, plants and animals, the text examines water stress at all levels of biological organization. Topics covered include: 1) organic osmotic agents: their distributions, modes of action, and mechanisms of regulation; 2) desiccation stress; mechanisms for preserving cellu lar integrity under conditions of low cellular water activity; 3) water stress and water compartmentation in plants; and 4) freezing stress: the prevention and regulation of ice formation in biological fluids, and mechanisms for overcoming the damaging effects of low temperatures on cellular integrity. Common adaptive strategies in diverse organisms are emphasized, as well as the fundamental physical-chemical properties of aqueous solutions that establish the nature of the interactions among water, low molecular weight solutes and macromolecules.