Knut Schmidt–Nielsen, born in Trondheim, Norway, was the leading comparative physiologist of the second half of the twentieth century. He played a key role in the transformation of what, in the first half of the century, had been a well–regarded but essentially local approach to the subject begun by his mentor, August Krogh, into what is now an established, worldwide field. He was a pioneer in integratingfield and laboratory measurements and in recognizing the intimate relationship between an animal's environment and its physiological adaptations. He greatly broadened and provided perhaps the best support for Krogh's notion that mechanisms could often be best illuminated by shrewd choice of experimental material, in particular by using animals in which a particular function met an especially strong challenge. He also drew attention to the way in which the pattern of variation of some feature or function varied withsize—‘scaling’—could be used as yet another tool for elucidating function. He began work that integrated animal locomotion and physiology, recognizing that locomotion typically represented an extreme challenge for physiological systems. More than any other person, he changed the way in which physiology is taught to undergraduates in the USA and elsewhere: from a human–oriented perspective derived from its origin in medicine to a comparative outlook compatible with the rest of biology.