BACKGROUND Severe carotid stenosis is the single most important aetiological factor in focal cerebral ischaemia. Carotid endarterectomy is indicated in stenosis of greater than 80% according to the results of the study ECST. METHODS Since the morbimortality associated with carotid endarterectomy in patients with moderate stenosis is similar to the incidence of stroke in patients treated with drugs, different criteria should be used to quantify stenosis for the detection of patients with a high risk of having a stroke. Ultrasound study (colour duplex) permits one to not only determine the degree of carotid stenosis, based on the characteristics of the flow curve, but also to analyse the structure of the atheromatous plaque. That is to say that one can distinguish between patients with a high risk of stroke and those with a low risk, independently of the degree of carotid stenosis. The ultrasound study has not only shown great precision in identification of the different components of the plaque of atheroma described in anatomopathological studies, but has also made apparent the association between certain characteristics of the atheromatous plaque, the risk of its progression and the risk of stroke. CONCLUSIONS In this review we consider the ultrasound characteristics of the atheromatous plaque and the risk of stroke associated with it; the different ultrasound patterns and their correlation with anatomopathological findings; we review the mechanisms of progression of atheromatous plaques and finally describe their classification according to the recommendations of the second International Consensus.