Cardiovascular diseases are the main cause of death worldwide, with coronary artery disease (CAD) being the predominant underlying etiology. Coronary angiography (CA) is the current invasive method used for CAD diagnosis, as well as for defining the coronary interventional treatment strategy. However, CA offers sometimes-poor accuracy in estimating atherosclerotic plaque volume, morphology and degree of stenosis severity. Optical coherence tomography (OCT) is an intracoronary imaging technique, developed in order to overcome CA limitations and is considered to be an "optical biopsy" that provides in vivo imaging. OCT has an extremely high resolution, similar to that of a usual histological evaluation of a biopsy sample. One of the most important clinical research areas for OCT is represented by the study of the pathophysiology of coronary and carotid atherosclerotic disease, in order to improve diagnosis and optimize therapy. This article reviews OCT basic technical aspects related to its diagnosis efficacy, OCT morphological information offered in coronary artery disease, including acute coronary syndromes and non-atherosclerotic coronary disease, OCT use for morphological percutaneous coronary intervention (PCI) follow-up and stent-failure mechanisms, as well as the new three-dimensional (3D)-OCT approach for atherosclerotic plaque assessment.