Acute coronary syndrome (ACS) encompasses conditions that include confirmed or suspected myocardial ischemia or myocardial infarction (MI). ACS, which includes non-ST-elevation MI (NSTEMI), ST-elevation MI (STEMI), and unstable angina, usually results from thrombus formation on a ruptured atherosclerotic plaque in the epicardial arteries. Patients most commonly present with chest pain but also may have atypical symptoms. Diagnosis is by clinical history, ECG changes, elevated cardiac enzymes (preferably, high-sensitivity cardiac troponin), and/or evidence of wall motion abnormalities on imaging. Both STEMI and NSTEMI have loss of myocardial tissue, but STEMI is due to transmural ischemia (indicating complete major coronary artery obstruction). NSTEMI occurs because of subendocardial ischemia (indicating partial major coronary artery obstruction). Unstable angina occurs when ischemia does not result in infarction (no troponin elevation and typically with non-ST-elevation changes on ECG). Management of STEMI depends on the timing of the presentation and local resources with regard to thrombolytic therapy versus percutaneous intervention. Both unstable angina and NSTEMI are managed the same way, with both conservative (medical) and invasive strategies available. Additionally, routine medical therapy includes dual antiplatelet therapy, nitrates, oxygen, pain control, and beta-blockers.