Fish have developed remarkable mechanisms for coping with life in water. The salinity/osmolarity of aquatic habitats can be quite variable. Fish have evolved mechanisms for maintaining fluid and electrolyte homeostasis across a wide range of salinities. Marine teleosts, freshwater teleosts, and marine elasmobranchs all utilize different physiologic strategies for osmoregulation. Kidneys do play a role in osmoregulation, but overall, extrarenal mechanisms are equally if not more important sites for maintaining osmotic homeostasis. Extrarenal sites include the gill tissue, the alimentary tract, the rectal gland (elasmobranchs), and the urinary bladder. Through an understanding of osmoregulatory physiology in these three groups of fishes, the effects of injury and disease on fluid and electrolyte balance can be elucidated. Armed with this knowledge and the results of diagnostic testing, safe and effective fluid/electrolyte support can then be instituted.