The respiratory muscles provide the motive power for breathing. Despite this central role in ventilation their physiology has been relatively neglected, perhaps partly because of the complexity of their function, and the difficulties of studying them. However, in the last decade, there has been considerable increase in interest, and a number of new concepts have arisen [l-31. It is now appreciated that the translation of central nervous output into ventilation requires a sophisticated integration of the respiratory muscles, which have to subserve the requirements of posture and body movement, simultaneously with breathing. The importance of the shape (configuration) of the respiratory system to muscle action has been emphasized. Analysis has shown that the functions of the diaphragm are complex: it is anatomically and embryologically derived from two muscles, and these parts may have different physiological actions. The abdominal muscles appear to be not only powerful muscles of expiration, but also facilitate inspiration. Both internal and external intercostal muscles now seem to be inspiratory at low lung volumes and expiratory at high lung volumes. Like all skeletal muscles the respiratory muscles are capable of fatigue after heavy loads. It is increasingly believed that respiratory muscle fatigue may play an important part in the pathogenesis of respiratory failure, and part of the value of artificial ventilation may lie in resting the respiratory muscles. The respiratory muscles comprise the diaphragm, the intercostal muscles, the abdominal muscles, and the so-called ‘accessory’ muscles including the sternomastoid and scalene muscles. However, probably all of the muscles of the trunk and neck can be recruited as respiratory muscles under heavy loads.