Exercise-induced asthma is considered in terms of the stimulus, the intermediary pathway and the response. Various controversies about each of these components are discussed. The stimulus may be cooling of the airways, loss of water or neither of these, and there is evidence for and against the identity of exercise- and hyperventilation-induced asthma. The intermediary pathway seems certain to involve the release of chemical mediators, although other neurogenic mechanisms have been proposed. Since the response is far from uniform, it may well be that different pathways are involved in different subjects. The effector mechanism appears to be bronchospasm, but recent evidence has suggested that an inflammatory response may be involved in a late reaction to exercise. The variability of exercise-induced asthma may well be due to variations in intrinsic bronchial reactivity resulting from allergenic stimulation.