This chapter provides the traditional and evolving criteria used for grading carotid artery stenosis as well as the clinical relevance of sonography in the management of symptomatic and asymptomatic carotid disease. Doppler diagnosis of carotid stenosis focuses on three areas: the prestenotic region, the stenosis itself, and the poststenotic region. Color-flow imaging permits rapid identification of the carotid vessels and allows for easier recognition of flow abnormalities that suggest the presence of stenosis. There are numerous spectral criteria for classifying stenosis in the internal carotid artery (ICA). Some focus on categories of stenosis, while others focus on threshold levels of stenosis. Carotid duplex studies have been used as the noninvasive standard to evaluate for carotid stenosis. Standardized criteria evaluating peak systolic velocities, end diastolic velocities, and the ICA/common carotid artery (CCA) ratio have been set and correlate to specific percentages of stenosis.