Water is essential for life. Water is the medium in which all biochemical reactions necessary for physiological functioning occur.

Water has unique properties that make it a universal solvent for biologically important solutes (compounds that are dissolved in water, including salts). The range of organismal water content conducive to physiological functioning is relatively narrow, between 60% and 90% of body mass.

In addition to maintaining a suitable water balance, organisms must also balance the uptake and loss of solutes, primarily salts. Salt balance is closely tied to water balance because the movements of water and salts influence each other. Aquatic environments may be more saline than an organism's cells or blood (hyperosmotic; hyper, “greater”), of similar salinity (isoosmotic; iso, “same”), or less saline (hypoosmotic; hypo, “less”). Most marine invertebrates rarely face problems with water and solute balance, because they tend to be isoosmotic.

Terrestrial organisms face the problem of losing water to a dry atmosphere, while freshwater organisms may lose solutes to, and gain water from, their environment. The evolution of freshwater and terrestrial organisms is very much a story of dealing with the need to maintain water balance. In this section, we will review some basic principles related to water and solute balance and provide some examples of how freshwater and terrestrial organisms maintain a water balance that is conducive to physiological functioning.