Complications of vulnerable atherosclerotic plaques (rupture, luminal and mural thrombosis, intraplaque hemorrhage, rapid progression to stenosis, spasm, and so forth) lead to heart attacks and strokes. It remains difficult to identify what plaques are vulnerable to these complications. Despite recent developments such as thermography, spectroscopy, and magnetic resonance imaging, none of them is approved for clinical use. Intravascular ultrasound (IVUS), a relatively old yet widely available clinical tool for guiding intracoronary procedures, is increasingly used for characterization of atherosclerotic plaques. However, inability of IVUS in measuring plaque activity limits its value in detection of vulnerable plaques. In this review, we present new information suggesting that microbubble contrast-enhanced IVUS can measure activity and inflammation within atherosclerotic plaques by imaging vasa vasorum density. An increasing body of evidence indicates that vasa vasorum density may be a strong marker for plaque vulnerability. We suggest that a combination of structural assessment (cap thickness, lipid core, calcification, etc) and vasa vasorum density imaging by IVUS can serve as the most powerful clinically available tool for characterization of vulnerable plaques. Due to space limitations, all IVUS images and movies are posted on the website of the Ultimate IVUS Collaborative Project: http://www.ultimateivus.com.