Intravascular ultrasound is an emerging imaging modality that employs miniaturized transducers at the tips of catheters to provide detailed information about the morphology of small and large arteries. While there is ongoing development in technology, currently available intravascular ultrasound instrumentation already augments angiography in the assessment of structural and dynamic abnormalities of the peripheral and coronary arterial systems. Recent studies have consistently demonstrated significant underestimation of atherosclerotic disease by angiography. Demonstration by intravascular ultrasound of disease in the angiographically normal "reference" segment and of remodeling of the diseased segment may explain this angiographic underestimation. Intravascular ultrasound also has many applications during various catheter-based therapeutic procedures. In addition to contributing to our understanding of the mechanisms of interventions and restenosis, intravascular ultrasound may lead to better guidance and ultimately better outcome during various procedures such as balloon angioplasty, atherectomy, and stent placement.