Despite the advent of sophisticated and expensive cardiologic tests, the electrocardiogram (ECG) remains the most reliable and inexpensive tool for the confirmation of acute myocardial infarction (MI), the commonly occurring atrial fibrillation and a host of other diagnoses not made by other advanced technologies. The clinical diagnosis of pericarditis or myocardial ischemia can be confirmed only by ECG findings. The ECG has no rival for the diagnosis of arrhythmia, which is one of the most common on-call problems in cardiology. This chapter covers the genesis of the QRS complex, a method for ECG interpretation, left ventricular hypertrophy, right ventricular hypertrophy, acute myocarditis, Non-ST elevation infarction, ischemia, Prinzmetal’s angina, pericarditis, hypokalemia ECG hallmarks, hyperkalemia, pulmonary embolism, arrhythmias, atrial premature beats, ventricular premature beats, narrow QRS tachycardia, atrioventricular nodal reentrant tachycardia, atrial tachycardia, Wolff-Parkinson-white syndrome, ECG clues during tachycardia, carotid sinus massage or intravenous adenosine, and bradyarrhythmias.