The Third Universal Definition of myocardial infarction (MI) and the clinical use of high-sensitivity troponins have resulted in an increase in patients recognized with a diagnosis of MI. Although the most common cause of MI remains acute coronary syndrome (ACS) caused by atherosclerotic coronary artery disease, it is increasingly important to be aware of other causes of ACS, which are likely to be seen with greater frequency as recognition of MI increases. It is essential to define the cause of ACS resulting in MI as it has profound implications for treatment strategy and prognosis.