Collateral coronary arteries provide alternate routes for delivery of blood to myocar­ dium distal to major coronary-artery obstructions. These channels may be extracar­ diac, endomural, or arterioluminal in origin (I). One additional source of collateral myocardial blood flow is from deliberate attempts to surgically augment blood flow to ischemic regions of myocardium. Over the years these have included methods such as promotion of adhesion formation with inflammation and epicardial in­ growth, direct intramyocardial tunneling of bleeding arterial vessels, and, most recently, the construction of aortocoronary interposition saphenous-vein bypass grafts. This review deals primarily with intercoronary collateral vessels, since these constitute the major and best studied class of naturally occurring collateral arteries. These arteries have an anatomic point of origin termed the "stem," a mid-zone, and a reentry point. The presence, number, and transmural distribution of coronary collaterals appear to be determined genetically for different species and perhaps differences in collateral density within a given species may also be a hereditary characteristic (2, 3). For example, dogs form extensive epicardial collaterals, while pigs have primarily a dense subendocardial network of collateral arteries. In con­ trast, collateral vessels in man have been demonstrated at every depth of myocar­ dium, but subendocardial connections appear most numerous. Goats, however, do not generally form collateral vessels (4, 5). Gradual closure of a coronary artery in the canine model rarely results in myocardial infarction, but similar preparations in pigs invariably result in some degree of tissue necrosis. The same technique applied to a goat usually results in the death of the animal from massive myocardial necrosis (6). Although man may form abundant collateral channels, it is unpredicta-