Duplex scanning of the carotid arteries combines B-mode ultrasound and Doppler ultrasound within a single instrument, thus providing both anatomical imaging of the vessel and flow velocity information. Combining these two unique aspects of ultrasound overcomes many of the individual limitations of each and provides improved noninvasive diagnosis of cerebrovascular disease. The Doppler signal is analyzed using fast Fourier transform methods and displayed as a spectral waveform. Assessment of the presence and severity of vascular disease is based primarily on Doppler information and the B-mode image of the vessel serves mainly as a road map to guide the placement of the pulsed Doppler sample volume. The parameters used to classify severity of disease include peak systolic frequency, degree of spectral broadening, end diastolic velocity, and overall shape of the waveform. Using these features, stenosis at the carotid bifurcation is classified into six categories: (A) none, (B) 1%-15% diameter reduction, (C) 16%-49% diameter reduction, (D) 50%-79% diameter reduction, (D+) 80%-99% diameter reduction, and (E) complete occlusion. These methods have a sensitivity of 99% and a specificity of 84% when compared with angiography. The clinical applications of carotid artery duplex scanning are discussed with particular reference to (1) patients with cerebrovascular symptoms, (2) asymptomatic cerebrovascular disease, (3) follow-up of carotid endarterectomy, and (4) cerebrovascular research. Patterns of use of duplex scanning in current practice and potential future applications are discussed.