This chapter focuses on the measurements of basic intervals, refractory periods and programmed electrical stimulation. Electrophysiological study (EPS) is performed most frequently as a part of ablative therapy for cardiac arrhythmias. The procedure is performed in a cardiac catheterization lab and needs the standard fluoroscopic equipment. Additional electrophysiology (EP) lab equipment including an oscilloscopic recorder, junction box, and a stimulator for delivering stimuli are also needed. The protocol and sequence of stimulation in the EP lab is dictated by the indication for which the EP study is being done. The study typically begins with the recording of a 12-lead ECG and intracardiac electrograms from the high right atrium, His bundle region, coronary sinus (CS) and the right ventricle. The CS catheter may not be needed in patients who are undergoing the study for syncope evaluation, bradycardia assessment or risk stratification of sudden death. Programmed electrical stimulation (PES) is defined as cardiac stimulation by a pulse of electrical current delivered by a stimulator through an electrode catheter that is in contact with tissue of a cardiac chamber using prespecified protocols. Programmed electrical stimulation is also described in this chapter.