Optical coherence tomography (OCT) has revolutionized intracoronary imaging. The unprecedented spatial resolution of this technique (15 μm) provides unique insights on the microstructure of the coronary wall. Currently, OCT is increasingly used in clinical practice and also constitutes an emerging, highly robust, research tool. OCT allows detailed visualization of atherosclerotic plaques and provides reliable information on plaque composition (lipid, fibrous, calcified) although its limited tissue penetration usually precludes a comprehensive analysis of the total plaque burden. OCT is the only technique allowing accurate measurements of the thickness of the fibrous cap, a classical marker of plaque vulnerability, and readily detects thin-cap fibroatheromas. In patients with acute coronary syndromes, plaque ruptures, with associated red or white thrombus, are nicely identified. OCT is also valuable to assess the results of coronary interventions. Stent expansion can be easily ascertained. In addition, due to its 10-times higher resolution, OCT is superior to intravascular ultrasound in the detection of even minor degrees of strut malapposition, tissue prolapse, residual thrombus and edge dissections. Furthermore, during follow-up OCT has a unique value to unravel the presence of strut coverage and to detect mild amounts of neointimal proliferation that might represent a valid surrogate marker of drug-eluting stent safety and efficacy. Finally, OCT has been used to unravel the underlying mechanisms implicated in stent failure, namely in-stent restenosis and stent thrombosis. Therefore, OCT appears ideally suited to help to move forward our understanding on the pathophysiology of coronary artery disease and to improve clinical decision-making processes, meeting the ever-increasing demand on coronary artery anatomical information from clinicians and researchers.