Intravascular ultrasonography has the capability to visualize the vessel wall and perivascular structures, to identify the spatial distribution and composition of atherosclerotic plaque, and to measure accurately vessel and wall dimensions. This article discusses relevant principles of instrumentation, issues pertaining to the validation and interpretation of intravascular ultrasound images, and the current and potential applications of intravascular (and intracardiac) imaging. Emphasis is placed on uses that are likely to be of interest to the critical care physician.