Coronary thrombosis is a major cause of acute myocardial infarction and sudden death in adults. Thrombi are the cause of the majority of acute infarcts, but are detected in only one-half of sudden death victims who die from severe coronary artery disease. The most common substrate of acute coronary thrombosis is plaque rupture, in which necrotic core is exposed to the luminal blood by a break in a fibrous cap that is generally infiltrated with macrophages. A less common substrate for coronary thrombosis is the plaque erosion, in which case there is denudation of the endothelial lining in direct contact with the thrombus, and a smooth muscle cell-rich plaque. Both of these types of thrombi may result in acute myocardial infarction or sudden death, or they may heal spontaneously and result in plaque enlargement. The autopsy pathologist should carefully section epicardial arteries, with special attention to decalcification, sampling of the entire tree, and deeper levels of areas of suspected plaque disruption, to prevent underdiagnosis, accurately diagnose the type of thrombus, and interpret the likely clinical significance of the coronary lesion.