Optical coherence tomography (OCT) is a light-based imaging modality that can be used in biological systems to study tissues in vivo with near-histologic, ultrahigh resolution. The rationale for intravascular application of OCT is its potential for in vivo visualisation of the coronary artery microstructure.The principle is analogous to pulse-echo ultrasound imaging; however, light is used rather than sound to create the image. Low-coherent near-infrared light is emitted by a superluminescent diode and reflected by the microstructures within biological tissues. The echo time delay of reflected light waves is converted into a two-dimensional spatial image. The intensity of the reflected light waves is translated into an intensity map. Experimental studies confirmed the ability of intravascular OCT for plaque characterisation and accurate assessment of vascular structures that are close to the luminal surface. Preliminary clinical experience proved in vivo feasibility of intravascular OCT. A variety of atherosclerotic plaque structures including thin cap fibroatheromas can be visualized in vivo.Intravascular OCT allows for accurate assessment of vessel structures close to the luminal side. Clinical application is feasible. To date, however, the clinical relevance of OCT findings in coronary arteries is unclear and further validation of OCT imaging is mandatory.