Working capacity is the physiological key to understanding man's ability, in technically less advanced communities, to exploit his environment, and hence to understanding his role in the ecological balance. In this volume the present state of knowledge of working capacity in tropical populations is reviewed in a series of illustrative papers. Topics cover the measurement of working capacity in populations: the functional consequences of malnutrition; growth, size and muscular efficiency; ethnic differences in working capacity; energy; expenditure and endemic disease; and energy flow in tropical ecosystems. These papers and their ensuing discussions lead to a series of recommendations on studies to be incorporated in the Decade of the Tropics research programme of the International Union of Biological Sciences.