Over the last decade, it has become progressively clear that the most important mechanism responsible for acute coronary and cerebrovascular events is atherosclerotic plaque rupture with superimposed thrombus formation. Anatomo-pathological studies have shown that the risk of rupture depends on plaque type rather than plaque size. The determinants of carotid plaque vulnerability to rupture are similar to those responsible for coronary instability: 1) size and consistency of the lipid-rich atheromatous core, 2) ongoing inflammation and repair processes within the fibrous cap, and 3) the thickness of the fibrous cap covering the core. Unstable plaques contain a soft, lipid-rich core that is covered by a thin and inflamed cap of fibrous tissue. External factors such as mechanical and hemodynamic stresses may be important not only in precipitating disruption of vulnerable plaques, but also in their cellular differentiation. Several imaging techniques have been used to identify plaques at high risk of events. High-resolution B-mode ultrasound is a noninvasive, inexpensive technique which allows a characterization of carotid plaque dimension, internal structure and surface. Nevertheless, such a method is not perfect. The subjective evaluation of plaque morphology on B-mode ultrasound, the need of improving reproducibility and the lack of a uniform terminology are critical issues, which need to be addressed.