One of the objectives of investigations carried out within the Human Adaptability section of the International Biological Programme was to obtain comparable data on population characteristics over a wide range of ecosystems. This volume provides a brief survey of the variables affecting the physiological work capacity of various populations. The items considered include body weight, fatness, maximum oxygen intake, muscular strength, and the process of oxygen transport from the air to the working muscles. The discussion is based largely on data collected under the auspices of the IBP, using methods standardised for the programme. The influence of race, heredity, environment and disease are considered, and a detailed analysis is made of various classes of athlete. The material will be useful to human and environmental physiologists, anthropologists, and those interested in physical education.