This book provides a detailed account of the intricate mechanisms and regulation of epithelial chloride transport, offering valuable insights into the regulation of salt balance in various vertebrate epithelial tissues. Over sixty years ago, J. Wendell Burger discovered that the rectal gland of elasmobranchs was the extrarenal organ responsible for excreting the salt gained by the animals. Since then, the rectal gland of elasmobranchs has been the only organ enabling whole organ, tissue, cellular and molecular studies of secondary active transport. The rectal gland has not only illuminated the mechanism of chloride transport, but also allowed the characterization of some of the proteins involved in transport. The NaK-ATPase was first isolated and purified, and its biochemical properties were linked to transport activities. The sodium-potassium-two chloride cotransporter NKCC1 was cloned from this gland. Understanding chloride transport, as revealed through experiments on this gland, has provided insight into the physiology and pathophysiology of chloride transport disorders. Readers will gain an insight into the scientists, protocols and specific experiments involved in nearly a century of research into the elasmobranch rectal gland, shedding light on the complex mechanisms and regulation of epithelial chloride transport.