Catheter-based intravascular ultrasound is a relatively new imaging tool to examine endovascular structure. One major goal for the development of intravascular ultrasound imaging has been to help clarify the mechanism of interventional therapies such as balloon angioplasty and directional atherectomy. Pathologic studies have suggested that plaque distribution and composition are key features that relate to initial and long-term success of coronary interventions. However, relatively little is known by angiography about the nature of plaque in the clinical setting. Intravascular ultrasound imaging provides a high resolution, "on-line" method of tracking the effects of catheter interventions such as balloon angioplasty and atherectomy. Because of its ability to visualize tissue beneath the luminal surface, ultrasound is generating new insights into the effect of plaque composition and distribution on the response to catheter therapies. With pre-procedure ultrasound imaging, it is increasingly possible to predict the result of a particular intervention, offering the potential for developing strategies of lesion-targeted therapy based on certain plaque characteristics. This article presents the early results of the GUIDE trial as a "works in progress" view of the role of intravascular ultrasound in helping interventionalists to understand--and to optimize--angioplasty and atherectomy.