Coronary artery disease represents a major cause of morbidity and mortality around the world. Unstable angina pectoris is a serious manifestation of ischemic heart disease and represents an acute condition caused by the narrowing of the coronary lumen as the result of an atheromatous plaque formation. In most cases the trigger of this process is represented by the rupture of a plaque that has become vulnerable or unstable. The first-line intracoronary imaging technique for the evaluation of plaque vulnerability is optical coherence tomography, which can measure the thickness of the fibrous cap (a significant predictor of plaque vulnerability) and can also assess other characteristics of plaque vulnerability (macrophage infiltration, lipid pool, intracoronary thrombus, or neointimal rupture). We present the case of a 67-year-old male with symptoms suggestive of unstable angina pectoris, caused by the presence of a vulnerable plaque on the left main coronary artery, where optical coherence tomography had a significant contribution in identifying the etiology of chest pain.