The systematic use of coronary angiography and the availability of increasingly sensitive biomarkers of myocardial cytolysis have determined an increase in the diagnosis of myocardial infarction with non-obstructed coronary arteries (MINOCA), which is currently reported in 5-20% of patients with acute myocardial infarction. The first diagnostic criteria of MINOCA were reported by a position paper of the European Society of Cardiology in 2017; since then, these criteria have been revised several times until the current version. The diagnosis of MINOCA requires not only the presence of myocardial damage, indicated by the detection of a rise or fall in cardiac troponin with at least one value above the 99th percentile upper reference limit, but also the clinical evidence of acute myocardial infarction as shown by symptoms, ECG, or imaging. This definition resulted in the exclusion from the umbrella group of MINOCA of certain conditions, such as takotsubo syndrome and myocarditis, that do not have a clear ischemic etiology. The term MINOCA encompasses heterogeneous conditions from a pathophysiological, clinical and therapeutic perspective. For this reason, MINOCA should be used to identify a "working diagnosis", which is the first step of a diagnostic work-up aimed at clarifying the mechanisms and identifying the most appropriate therapy for the individual patient. The aim of this review is to describe the state of the art regarding the definition, classification, and diagnosis of MINOCA, providing an excursus on the principal documents proposed by scientific societies or experts in the field in recent years.