Intravascular ultrasound (IVUS) is a valuable adjunct to angiography, providing new insights in the diagnosis of and therapy for coronary disease. IVUS allows tomographic assessment of lumen area, plaque size, distribution, and composition. The safety of IVUS is well documented, and the assessment of luminal dimensions represents an important application of this modality. Intravascular Ultrasound (or IVUS) allows us to see a coronary artery from the inside-out. This unique point-of-view picture, generated in real time, yields information that goes beyond what is possible with routine imaging methods, such as coronary angiography, performed in the cath lab,or even non-invasive Multislice CT scans. Intravascular ultrasound is done in the catheterization laboratory in conjunction with angiography. Some cardiologists use it occasionally, in difficult cases, or to assist in the selection and sizing of stents and balloons. IVUS commonly detects occult disease in angiographically normal sites. In ambiguous lesions, ultrasound permits lesion quantification, particularly for left main coronary diseases.