Acute coronary syndrome consists of acute myocardial infarction associated with electrocardiographic ST segment elevation (STEMI) and unstable angina/non-ST segment elevation myocardial infarction (UA/NSTEMI). Theses three conditions are differentiated primarily by clinical, electrocardiographic, and laboratory presentations correlating with the duration and extent of thrombotic occlusion as a consequence of plaque rupture. Acute coronary syndrome is now the most prevalent presentation of ischemic heart disease in Korea as well as developed countries. Q-wave acute myocardial infarction is the most easily identified syndrome ; persistent angina and ST-segment elevation are characteristic of sudden total or near-total thrombotic arterial occlusion. Urgent reperfusion is the mainstay of therapy, whereas UA/NSTEMI is usually associated with severe coronary obstruction but not total occlusion of the culprit coronary artery. The distinction between unstable angina and non-Q wave, non-ST segment elevation myocardial infarction is often not clear at the initial presentation of patients. Unstable angina is defined as angina pectoris with at least one of the following three features : ① It occurs at rest usually lasting more than 20 minutes. ② It is severe and described as frank pain and of new onset within 1 month. ③ It occurs with a crescendo pattern (more severe, prolonged, or frequent than previously). Some patients with this pattern of ischemic discomfort, especially those with prolonged rest pain, develop evidence of myocardial necrosis on the basis of the release of cardiac markers and thus have a diagnosis of NSTEMI. This diagnosis has been based on the elevation of serum creatine kinase (CK)-MB, level but recently troponin T and I assays are used to define ischemic myocardial damage based on their higher sensitivity for myocardial necrosis and prognostic implication. Troponin has become the standard myocardial marker. A certain number of patients have elevated levels of troponins in the absence of significant CK elevation. Several studies have shown that despite nondiagnostic CK levels, an elevated troponin level often indicates a poor prognosis.