This chapter was intended to provide an overview of ultrasound duplex scanning at the human carotid bifurcation. Although results of clinical studies are useful in patient management, much remains to be learned in the area of understanding the disease process and determining what features, obtainable by ultrasound, will predict clinical outcome. Listed below are the important points covered by this chapter. 1. Ultrasound duplex scanning can detect and quantify carotid artery disease with respect to the gold standard of contrast arteriography. 2. Contrast arteriography is not the ideal gold standard for assessment of cartotid artery disease. A new standard incorporating attributes of not only anatomy but also physiology is needed. 3. Using currently available ultrasound systems it is not possible to obtain the Doppler angle (i.e., the angle formed by the transducer axis and the blood flow-velocity vector). This angle is often estimated by the angle formed by the transducer axis and the axis of the blood vessel. This assumes that the flow-velocity vectors are axial, which turns out to be a poor assumption at bifurcations, curves, and sites of intraluminal disease. Specifying the Doppler angle involves assumptions that should always be kept in mind. 4. All Doppler data should be collected at the same angle formed by the sound beam relative to the vessel axis. Whereas results of Doppler studies can be reported in units of either frequency or velocity, assumptions made about the Doppler angle and acquiring data at arbitrary angles can produce large errors when quantifying flow velocities. 5. Blood flow disturbances cannot automatically be equated with the presence of disease. Some disturbances are localized secondary blood flow patterns produced by changes in vascular geometry coupled with pulsatile flow. 6. The lack of blood flow disturbances cannot automatically be equated with a normal or disease-free artery. Accumulation of plaque within the sinus region at the minimal and moderate disease stages appears to eliminate or attenuate local flow disturbances characteristic of a normal bifurcation. 7. An understanding of the blood flow patterns at clinical sites of interest is important to the correct interpretation of data. Both the imaging and pulsed Doppler modalities are essential to this understanding: the imaging capability to place the pulsed Doppler sample volume and appreciate changes in vascular anatomy, and the pulsed Doppler to obtain the character of blood flow at specific sites within the blood vessels.