This chapter presents an overview of the ionic and osmotic strategies of the major fish groups, reviews the structure and function of iono- and osmoregulatory tissues, and considers the challenges faced by fishes that encounter environmental salinity fluctuations in space and/or time. Euryhaline elasmobranchs are iono- and osmoregulators in freshwater (FW), with high body fluid osmolality due to urea and trimethylamine oxide concentrations that are reduced in FW but remain considerable. The main iono- and osmoregulatory tissues are skin, gill, renal, and gastrointestinal epithelia. The exceptions include specific skin regions that are ionocyte rich, such as the opercular epithelium, and a few marine and amphibious species that have ionocytes across the body surface. The branchial epithelium of marine teleosts contains an NaCl-secreting ionocyte. Ionocytes differentiate into distinct populations depending on the external salinity. Fishes exhibit a spectrum of strategies ranging from iono- and osmoconforming to iono- and osmoregulating.