There is a circulatory network in the heart consisting of multiple small-diameter vascular structures between the coronary arteries, connecting the coronary arteries to each other. These vascular structures which are the precursor of coronary collateral circulation are not visible in those having normal coronary arteries or mild-to-moderate coronary artery occlusions because they are too small in diameter to be seen on coronary angiograms and also have minimal blood flow. They have been observed only in postmortem studies. The sensitivity of tissues to ischemia is important as well as the extent and severity of atherosclerosis. The presence of well-developed coronary collateral circulation renders the myocardial tissue more resistant to ischemia and the patients less symptomatic. Considering the fact that coronary collateral circulation develops in response to severe coronary artery disease, it remains unknown why there are major differences among patients with the same degree of coronary artery disease and which factors are responsible for these differences.