Quantification of coronary artery calcium by computed tomography is used to detect coronary atherosclerosis in both symptomatic and asymptomatic individuals. It correlates with the extent of atherosclerosis in individuals with obstructive and nonobstructive coronary artery disease. It is unique among the various noninvasive techniques available to assess coronary artery disease, which typically depend on the presence of flow-limiting lesions. It appears to be a better predictor of risk of future events than conventional risk factors because it is anatomical evidence of subclinical disease. To best use this technology, an understanding of the pathologic principles underlying coronary artery calcification and new concepts in the theory of atherosclerosis progression are necessary. These concepts are true whether helical or electron beam computed tomography is used to quantitate coronary artery calcium.