This volume is the product of a February 1982 conference, cosponsored by the American Heart Association, the National Institutes of Health, and the Bowman Gray School of Medicine, which examined techniques for delineating quantitatively the natural history of atherosclerosis. Against the background of current pathologic and clinical knowledge of atherosclerosis, invasive and noninvasive evaluative methods now in use and under development are surveyed in depth. Correlative clinicopathologic studies of atherosclerosis pose special questions with respect to both luminal and plaque characteristics that are addressed in this volume. An old observa- tion, based on the examination of arterial casts, suggested that the so-called nodose lesion of atherosclerosis may be at first flattened into the wall of a weakened, dilated artery, rather than raised into the lumen. This is now fully confirmed in vivo by ultrasonic and other imaging techniques. The morbid anatomist is challenged anew to describe lesions as they are likely to occur in vivo. To achieve closer correlation with natural conditions, perfu- sion fixation of arteries under arterial pressure is becoming more widely used and has already demonstrated more valid quantita- tion of the composition and configuration of lesions. While the noninvasive methods of B-mode and Doppler ultrasound are suitable only for the clinical study of superficial arteries, such as the carotid or femoral, the new and relatively noninvasive procedure of intravenous digital subtraction angio- graphy can be effectively used for the examination of deep systems, such as cerebral vessels.