This report is a concise review of current knowl-edge of the structure and function of the intima of the aorta and the major distributing arteries. The main purpose of the review is to delineate normal arterial intima from atherosclerotic lesions and, in particular, to distinguish physiological adaptations from atherosclerotic increases in intimal thickness. To characterize normal intima, including the adaptive intimal thickenings, some of which represent locations in which atherosclerotic lesions are prone to develop, the structure, composition, and functions of the arterial intima in young people as well as in laboratory animals not subjected to known atherogenic stimuli are reviewed. This report on arterial intima is the first in a series of four. The second report will review and define initial, fatty streak, and intermediate types of atherosclerotic lesions, and the third report will review all types of advanced (ie, potentially clinical and clinical) lesions. The overall objective is to define arterial intima and all types of atherosclerotic lesions, and then to postulate, in a fourth and final report, a valid and up-to-date pathobiologicaJ nomenclature and classification of atherosclerotic lesions.