In the United States, more than 300,000 sudden cardiac deaths (SCD) occur each year, the most common underlying etiology of which is coronary heart disease (CHD). With increasing age, the proportion of cardiovascular causes among all natural sudden deaths increases, and the fraction of CHD deaths that are sudden decreases. The lower incidence of coronary atherosclerosis in women is reflected in their proportionately lower risk of SCD. There are no data suggesting specific hereditary factors in risk for SCD due to CHD. On an individual basis, the known risk factors for CHD cannot distinguish those at risk for SCD from those at risk for other manifestations of CHD. Pathologic studies of SCD victims have shown that extensive coronary atherosclerosis is a major marker; healed myocardial infarction is a common finding, as is myocardial hypertrophy. Nonatherosclerotic coronary artery abnormalities may be associated with SCD. Chronic congestive heart failure is a common cause of SCD. Other etiologies include inflammatory, infiltrative, neoplastic and degenerative processes, diseases of the cardiac valves, and primary electrophysiologic abnormalities. The largest group of prehospital cardiac arrest victims have ventricular fibrillation identified on initial contact, with their survival outcome being intermediate between patients with ventricular tachycardia (best outcome) and those with bradyarrhythmia or asystole (worst outcome) on initial contact.