The 12-lead ECG remains a simple and inexpensive technique to diagnose AMI in its early phases. The diagnostic accuracy of the ECG depends upon the extent of myocardial necrosis and its localization. The ECG is most sensitive in patients with occlusion of the LAD artery, followed by the RCA and the left CFA. In 10% to 20% of patients with AMI the initial ECG either shows nonspecific changes or is normal. The correlation between the ECG and infarct-related artery varies according to the involved vessel. Classic ECG changes are seen in 90% of the LAD artery, in 70% to 80% of RCA, and in only 50% of CFA occlusions. A second important issue is the mechanism and clinical significance of reciprocal ST segment changes, which usually indicate larger MI, more impaired ventricular function, worse prognosis, and in some patients, significant disease of a noninfarct-related artery. Furthermore, the value of the ECG in estimating myocardial injury and infarct size remains controversial. The ECG plays an important role in coronary reperfusion. ST segment elevation is one of the principal criteria for instituting thrombolytic therapy, and helps predict those who will most likely benefit from coronary reperfusion. The role of the ECG in evaluating the reperfusion status after coronary thrombolysis is not clear. Rapid return to baseline or normalization of the ST segment suggests opening of the occluded vessel, though a small or negligible change does not exclude successful reperfusion.