Intravascular ultrasound (IVUS) represents the gold standard in the assessment of atherosclerotic disease. It has deeply affected our understanding of coronary artery disease and therapeutic strategies. We learned that a phenomenon described in pathologic series plays a very important role during life--that of vessel remodeling. The type of remodeling (positive or negative) cannot been assessed by angiography but is clinically relevant for optimal, procedural lumen gain. The modern stent implantation technique with a high balloon to artery ratio and high balloon pressure is the result of the IVUS finding that the majority of stents are not optimally deployed despite appropriate angiographic results. IVUS is essential in the clinical practice for the assessment of ambiguous angiography, left main stem disease, complex lesion and in-stent restenosis. Furthermore, IVUS has become an indispensable research tool in the analysis of new therapeutic strategies such as coronary brachytherapy.