Although angiography is widely applied in the diagnosis of the coronary artery disease (CAD), studies have questioned the accuracy of radiographic methods. Miniaturized intravascular ultrasound devices offer several potential advantages, including a tomographic orientation and the ability to characterize atherosclerotic plaques. Two dissimilar technical approaches to transducer design have emerged: mechanically rotated devices and multi-element arrays, each yielding small coronary catheters (1.1-1.8 mm in diameter). Initial studies of coronary ultrasound have demonstrated few serious untoward effects. In most normal subjects, the vessel wall is laminar in appearance with an intimal leading edge and subintimal sonolucent layer averaging less than or equal to 0.20 mm in thickness. In most CAD patients, the thickness of the leading-edge or sonolucent zone is abnormally increased. Atherosclerotic abnormalities are frequently evident in segments with no angiographic lesion. At sites with a circular lumen shape, minimum diameter by ultrasound and angiography correlate closely, r = 0.93. At sites with an eccentric lumen, significant disagreement between angiography and ultrasound diameter is evident, r = 0.78. Correlation between angiography and ultrasound from percent stenosis is moderate, r = 0.63. Following balloon angioplasty percutaneous transluminal coronary angioplasty (PTCA), morphologic findings include complex cracks, splits, and dissections, and minimum lumen diameter by angiography and intravascular ultrasound correlate poorly. Recent advances have improved the utility of coronary ultrasound, including smaller catheters and a device combining a miniature imaging transducer (1.16 mm) with a low profile balloon (0.028-0.033 inch). Important current limitations include inability to visualize the smallest coronaries and tight stensoses. The future of coronary ultrasound is promising, propelled by the unique capability of this modality to image atherosclerotic plaques directly.