August Krogh, the son of a brewer, studied zoology in Copenhagen and earned his doctoral degree under the physiologist Christian Bohr, the father of the world-renowned nuclear physicist Niels Bohr. Krogh's unusual ability to construct instruments and complex apparatuses and his intuitive understanding of physical principles made it possible for him to improve on Bohr's methods. His findings led him to challenge Christian Bohr's ideas about oxygen secretion, and when Bohr refused to accept his findings, Krogh unwillingly came into a painful conflict with his own mentor. Krogh's continued studies of how oxygen is supplied to the tissues led to his realization that the blood flow in the finest blood vessels, the capillaries, has to be regulated through a mechanism that opens and closes the capillaries according to the tissue's need for oxygen. This idea and his scientific proof were at the time so new and revolutionary that he was promptly (in 1920) awarded the Nobel Prize. His fame in Denmark and all over the world continued to grow until his death in 1949. His scientific discoveries extended from respiration, exercise physiology and capillary physiology into comparative osmoregulation, isotope studies, active transport of ions in plants and animals, and finally insect flight. Another dramatic story of Krogh's life began when he introduced insulin production in Denmark in 1922. This move saved his own wife's life as well as numerous other lives and helped make Denmark's Novo-Nordisk the largest producer of insulin in the world today. Krogh's wife, Marie, became a physician and a renowned scientist in her own right. Throughout their harmonious marriage and partnership, Marie played an important role in her husband's life both scientifically and personally. Written by the proud daughter of August and Marie Krogh, this biography is based on numerous letters, scientific papers, interviews, symposia, and other sources as well as the author's own knowledge of her parents.