The purpose of an electrophysiological study (EPS) is to determine the mechanism of arrhythmias and to decide the means of therapy for that arrhythmia. The electrode catheters are introduced percutaneously into the atria or ventricle to record the intracardiac electrogram or to stimulate those areas by electrical current. In bradycardia, atrioventricular block and sick sinus syndrome are the good indications of EPS. To determine the indication of pacemaker implantation, recording the block site of atrioventricular block is useful, and sinus node recovery time and sinoatrial conduction time in sick sinus syndrome are important by performing an electrical stimulation to the atrial muscle. In tachyarrhythmia, if the mechanism of arrhythmia is reentry, it can be induced or terminated by electrical stimulation repeatedly. So that the drug efficacy can be defined by testing the re-inducibility after the drug therapy. If the mechanism of arrhythmia is not clear, the EPS is also performed to make the mechanism of the arrhythmia clear. In triggered activity, especially early after depolarization, the recording of monophasic action potential is useful for diagnosis. EPS can provide the ablation site clear, moreover, the indication of implantable cardioverter defibrillator has been decided by EPS. Therefore, the role of EPS has also been important in selection of non-pharmacological treatment.