Recent advances in microelectronic and piezoelectric technology have permitted development of miniaturized ultrasound devices capable of real-time tomographic intravascular imaging. Initial studies have successfully employed intravascular ultrasound to augment angiography in both diagnostic and therapeutic catheterization. The cross-sectional perspective of intravascular ultrasound appears to be ideally suited for precision measurements of luminal diameter and cross-sectional area. In addition, ultrasound improves assessment of problem lesions such as ostial stenoses or disease at bifurcations. Intravascular imaging provides unique, detailed cross-sectional images of the arterial wall not previously obtainable in vivo by any other technique and is more sensitive than angiography in detecting atherosclerosis. Intravascular ultrasound images of atherosclerotic wall abnormalities have the potential to augment greatly the understanding of the anatomy and pathophysiology of coronary disease. For interventional applications, ultrasound analysis of lesion characteristics offers many potential advantages. Evaluation of the normal reference segment used for device sizing constitutes an important emerging application for intravascular imaging. After the procedure, intravascular ultrasound often yields smaller luminal size measurements than angiography and greater of stenosis. These differences probably reflect augmentation of the apparent angiographic diameter by extraluminal contrast within cracks, fissures or dissection planes. New ultrasound instruments under development combine an imaging transducer with an interventional device, permitting on-line guidance during the procedure. Although the clinical value of routine ultrasound imaging before or after mechanical revascularization has not been tested by randomized trials, it seems likely that this new imaging modality will provide valuable insights into diverse phenomena such as abrupt occlusion and restenosis. Accordingly, we anticipate a continuing role for this modality in cardiovascular interventional therapy.