Amphibians were the first vertebrates to emerge from aquatic habitats, and modern species have evolved a variety of mechanisms to regulate water and electrolyte homeostasis. Fossil evidence for terrestrial vertebrates first appears in the upper Devonian (360–380 mya) in the form of tetrapods such as Ichthyostega and Ancanthostega. The fossil record has a large gap before the emergence of the major tetrapod lineages in the Upper Carboniferous with numerous amphibian taxa that are primarily associated with freshwater deposits. The earliest fossils that can be attributed to modern amphibians first appear in the early Triassic, and their relationship with the primitive forms is speculative. Modern amphibians are collectively termed Lissamphibia (smooth skinned) and include three orders: Anura (frogs and toads), Caudata/Urodela (newts and salamanders), and Gymnophiona (legless wormlike animals also known as apodans or caecilians). All three orders have species that occupy a wide range of habitats from purely aquatic to highly terrestrial. The phylogeny of Amphibia has recently been reevaluated by Frost et al. to include both morphological and molecular parameters. Many of the traditional genera have been divided to reflect a more detailed description of evolutionary relationships; for example, the North American frog Rana pipiens is now classified as Lithobates pipiens, whereas the European frog, R. temporaria remains in the genus Rana. We have elected to retain the traditional generic names because they are historically embedded in the literature cited in this chapter and still widely used by contemporary researchers in the field of ionic and osmotic regulation.