Background: Electrocardiogram (ECG) is based on changes of electrical currents of the heart (measured in millivolts). Standard calibration of the ECG is 10mm/mV. Therefore 0.1 mV equals to 1 mm square on the vertical axis. For simplicity, ECG deviations are expressed in mm following the standard calibration. It is recommended to initiate ECG monitoring as soon as possible in all patients with suspected STEMI in order to detect life threatening arrhythmias and allow prompt defibrillation if indicated and when a STEMI is suspected, a 12-lead ECG must be acquired and interpreted as soon as possible at the time of first medical contact to facilitate early STEMI diagnosis and triage. Objective: To give an overview about Electrocardiogram role in acute myocardial infarction. Conclusion: Acute myocardial ischemia is often associated with dynamic changes in ECG waveform and serial ECG acquisition can provide critical information, particularly if the ECG at initial presentation is non-diagnostic. Recording several standard ECGs with fixed electrode positions at 15 – 30 min intervals for the initial 1 – 2 h, or the use of continuous computer-assisted 12-lead ECG recording (if available) to detect dynamic ECG changes, is reasonable for patients with persistent or recurrent symptoms or an initial non-diagnostic ECG. ECG manifestations suggestive of acute myocardial ischemia: New ST-elevation at the J-point in two contiguous leads with the cut-point: ≥ 1 mm in all leads other than leads V2–V3 where the following cut-points apply: ≥ 2mm in men ≥ 40 years; ≥ 2.5 mm in men < 40 years, or ≥ 1.5 mm in women regardless of age. New horizontal or down-sloping ST-depression ≥ 0.5 mm in two contiguous leads and/or T inversion > 1 mm in two contiguous leads with prominent R wave or R/S ratio > 1.