therosclerosis remains the leading cause of morbidity and mortality in the Western world [1, 2]. It is usually regarded as a systemic disease and several well-identified risk factors (i.e., hypertension, hyperlipidemia, diabetes mellitus, and cigarette smoking) have been implicated in its pathogenesis. Noninvasive imaging of atherosclerosis, such as intimal wall thickening and plaque formation, is routinely available using a variety of imaging techniques. The causes of atherosclerosis are multifactorial and knowing them could allow earlier prevention and detection of the disease. The frequent occurrence of atherosclerotic plaques in well-recognized arterial districts, together with the focal distribution of these plaques in regions of curvature, bifurcation, and branching of the vessels, suggests that fluid dynamics and vessel geometry may play a localizing role in the cause of plaque formation [3]. In vitro and in vivo models have been used to study the flow patterns in a variety of arterial territories and to look for an association between areas of preferential atherosclerotic plaque formation and blood flow hemodynamics [4‐7]. The magnitude and rate of change of blood flow close to a vessel wall, which can be expressed using the concept of wall shear stress, have been linked to the pathogenesis of atherosclerosis. Vessel segments with low wall shear stress or highly oscillatory wall shear stress appear to be at the highest risk for development of atherosclerosis. The purpose of this review is to familiarize the imager with the concepts and principles underlying wall shear stress measurements. Wall shear stress mapping may someday become part of the multifactorial, multidisciplinary approach to early atherosclerosis detection. We first review this hypothesized association between atherosclerosis and wall shear stress. We then review the use of phase-contrast velocity-encoded MR imaging and Doppler sonography to map velocity profiles in vascular structures. These hemodynamic parameters can be used to calculate how quickly the blood velocity increases when moving from the vessel wall to the center of the vessel. Estimation of wall shear stress is now possible with noninvasive imaging techniques such as MR imaging and Doppler sonography. MR imaging can be used to confirm, in vivo, what is known from in vitro hemodynamic studies and observations made at autopsy and in experimental models. Someday, this may help us better understand the importance of flow hemodynamics in the multifactorial etiology of atherosclerosis. Wall Shear Stress