Coronary vasospasm plays a role in several manifestations of coronary heart disease, though its causes still remain to be established. Among suggestions made to account for it are normal increases in vasomotor tone, particularly in stenosed arteries, and local vascular hypersensitivity reactions. Stimuli known to precipitate spasm are discussed, particularly the role of the sympathetic nervous system and the possible involvement of platelet aggregation. Coronary vasospasm is stimulated by a number of different triggering mechanisms which vary between patients and even within patients at different times. However, it is concluded that vasospasm is the result of an interaction between hypersensitive vascular smooth muscle and a number of specific constrictor stimuli.