Most heart attacks are caused by Coronary Artery Disease (CAD). In coronary artery disease, a wax-like material called plaque builds up on the inside walls of arteries in the heart. This is called atherosclerosis. Plaque is made of cholesterol and other cells. The amount of plaque increases slowly. As more plaque builds up, the insides of the heart's blood vessels get narrower. This means that less blood can flow through the blood vessels. This can cause platelets (which make the blood clot) to build up in front of the plaque. This causes a blood clot in the blood vessel. If the clot breaks free and gets stuck in part of the blood vessel made narrower by the plaque, the plaque and the clot together block the blood vessel completely. This makes it impossible for blood to get to part of the heart, and causes a heart attack.