Coronary angiography (CAG) remains the gold standard for invasive assessment of coronary artery disease (CAD) and for guiding percutaneous coronary intervention (PCI). However, it is limited by its poor resolution, angle dependency, vessel overlap and fore-shortening and significant inter- and intraobserver variability. This chapter covers the principles and imaging systems, image interpretation, clinical applications of intravascular ultrasound, intravascular ultrasound in specific lesion subsets, and future perspectives. Intravascular ultrasound is used for almost all the PCI procedures in countries like Japan and South Korea. However, cost constrains and need for expertise limit its wide spread application in most of the catheterization laboratories. Intravascular ultrasound is used for almost all the PCI procedures in countries like Japan and South Korea. However, cost constrains and need for expertise limit its wide spread application in most of the catheterization laboratories.