It is often referred to as hardening of the arteries, but you can think of it as a series of deposits, or plaque, which gradually develop on the insides of the vessels, and which can eventually block them off completely.
In Western societies atherosclerosis is very common: about 70 percent of men and women over the age of 65 have deposits in their carotid arteries which can be detected by a simple noninvasive test.
Atherosclerosis starts in childhood, and gradually develops over 50 years or so before causing any problems. It's first detectable as streaks of fat in the inner lining of the arteries and then gradually enlarges. The plaque is not deposited uniformly, however, and there are certain favored spots, particularly where a large artery divides into two branches. Nothing happens until the plaque has narrowed the inside of the artery by at least 50 percent. When it gets to be more than that it starts to interfere with the blood flow through the artery, which is when the troubles start. If the obstruction first develops in one of the arteries supplying the heart (the coronary arteries), it is manifested as angina. More disastrously, a blood clot may form on the plaque, completely plugging the artery and leading to a heart attack.
