From the studies and observations reviewed in this chapter, it can be appreciated that there is considerable direct and circumstantial evidence to implicate coronary artery spasm as a primary phenomenon in some patients with myocardial infarction. Although currently available data indicate that the number of acute myocardial infarctions due primarily to "pure" coronary artery spasm is relatively small, it should be emphasized that essentially very little if any data exist at "time zero" or at the actual onset of infarction. Most myocardial infarctions are probably multifactorial in etiology. To what extent coronary artery spasm, atherosclerosis, platelets, in situ thrombosis, endothelial mediators, and passive influences on vascular caliber interact in the pathogenesis of acute myocardial infarction remains incompletely understood.