This chapter discusses the diagnosis, evaluation and management of acute coronary syndrome (ACS). ACS is a spectrum of disease and can present as acute myocardial infarction (AMI) or unstable angina (UA). There are four main elements in the diagnosis of ACS: clinical history, physical examination, electrocardiogram findings and cardiac biomarkers. Any patient with ST-segment elevation myocardial infarction (STEMI) should undergo reperfusion with percutaneous coronary intervention (PCI) within 90 minutes of presentation. Fibrinolytics should be used for patients unable to undergo PCI within the recommended timeframe. Beta-antagonists have been shown to benefit post-MI patients within 24 hours of the initial event when administered orally. ACE inhibitors are also recommended within 24 hours post event, but not in the immediate treatment of ACS. The three most common reasons for decompensation of the ACS patient include cardiac arrhythmias, cardiogenic shock with congestive heart failure, and mechanical complications.