Hans Ussing was born on 30 December 1911 at Sorø Academy in Denmark, where his father Dr Henrik Ussing was a lecturer and, as historian, a leading Danish folklorist. After his doctoral thesis in marine biology, Hans Ussing came to August Krogh's laboratory, where he studied protein turnover by using deuterium-labelled amino acids. After World War II, when radioactive isotopes of light elements became available for biological research, Ussing pioneered the development of epithelial physiology by introducing new concepts and theoretical tools, such as unidirectional fluxes, exchange diffusion, the flux-ratio equation, the shortcircuiting technique, solvent drag, anomalous solvent drag and the pre-steady-state flux ratio theorem. In studies on frog skin, combining electrophysiology and radioactive tracer technology, he provided the first unambiguous demonstration of active transport of sodium ions. His two-membrane hypothesis of active transport by frog skin initiated studies of epithelial transport at the cellular level in other organs and of the mechanisms of action of hormones and drugs. His discovery of paracellular ion transports bridged the physiology of high-resistance and low-resistance epithelia. With the Na⁺ recirculation theory of isotonic transport he continued his studies of epithelial physiology until shortly before his death. Ussing's scientific research provided analytical methods and new insights of general applicability for the study of absorbing and secreting epithelia-of equal importance to biology and medicine. Hans Ussing died on 22 December 2000 after a short illness.