August Krogh (1874–1949) was amongst the most influential physiologists in the first part of the 20th century. This was an era when physiology emerged as a quantitative research field and when many of the current physiological disciplines were defined; Krogh can rightfully be viewed as having introduced comparative physiology, epithelial transport and – together with Johannes Lindhard – exercise physiology as independent disciplines. With a unique ability to design and construct equipment, Krogh could address novel questions in both human and animal physiology with unprecedented precision. Krogh would characteristically focus on a given physiological problem over a couple of years, delineate the focal mechanisms, provide a solution to the major problems, and then move onto new academic ground. For each of his major research areas (respiratory gas exchange, capillary function, osmoregulation), he wrote comprehensive books or monographs that remain important resources for scholars today, and he engaged in the writing of physiology textbooks for the Danish high school. Krogh's research appears to have been driven by curiosity to understand how animals (including humans) work, but he did not hesitate to apply his insight to societal and clinical problems throughout his long academic career.