The Fourth Universal Definition of Myocardial Infarction (2018) was developed by the Joint European Society of Cardiology (ESC)/American College of Cardiology (ACC)/American Heart Association (AHA)/World Heart Federation (WHF) Task Force. It introduces a more comprehensive and accurate classification system for myocardial infarction (MI) and myocardial injury. The document defines MI as acute myocardial injury with clinical evidence of acute myocardial ischemia and elevated cardiac troponin (cTn) levels above the 99th percentile upper reference limit (URL). It also differentiates between various types of MI, including Type 1 (atherothrombotic), Type 2 (ischemia unrelated to atherothrombosis), Type 3 (death before biomarker testing), and procedure-related types (Type 4a, 4b, 4c, and 5). The document emphasizes the importance of high-sensitivity cardiac troponin assays for detecting MI and myocardial injury. It also addresses the role of imaging, ECG changes, and other clinical factors in diagnosing MI. The document outlines criteria for differentiating MI from myocardial injury and provides guidance on the management of patients with suspected MI. It also includes new sections on conditions such as Takotsubo syndrome, myocardial infarction with nonobstructive coronary arteries (MINOCA), and chronic kidney disease. The document aims to improve the accuracy of MI diagnosis and ensure consistent clinical management across different populations and settings.The Fourth Universal Definition of Myocardial Infarction (2018) was developed by the Joint European Society of Cardiology (ESC)/American College of Cardiology (ACC)/American Heart Association (AHA)/World Heart Federation (WHF) Task Force. It introduces a more comprehensive and accurate classification system for myocardial infarction (MI) and myocardial injury. The document defines MI as acute myocardial injury with clinical evidence of acute myocardial ischemia and elevated cardiac troponin (cTn) levels above the 99th percentile upper reference limit (URL). It also differentiates between various types of MI, including Type 1 (atherothrombotic), Type 2 (ischemia unrelated to atherothrombosis), Type 3 (death before biomarker testing), and procedure-related types (Type 4a, 4b, 4c, and 5). The document emphasizes the importance of high-sensitivity cardiac troponin assays for detecting MI and myocardial injury. It also addresses the role of imaging, ECG changes, and other clinical factors in diagnosing MI. The document outlines criteria for differentiating MI from myocardial injury and provides guidance on the management of patients with suspected MI. It also includes new sections on conditions such as Takotsubo syndrome, myocardial infarction with nonobstructive coronary arteries (MINOCA), and chronic kidney disease. The document aims to improve the accuracy of MI diagnosis and ensure consistent clinical management across different populations and settings.