**Abstract**

Although electrocardiography is more than 100 years old, it still holds the key to diagnose many disorders and is one of the most commonly used diagnostic tools not only in cardiology but throughout medicine. Most often a surface electrocardiogram (ECG) is made, which represents a summarized electrical activity of the heart. However, by inserting catheters into the heart, it is possible to make an ECG from different localized areas. This chapter focuses on introducing the readers to the world of cardiac electrophysiology providing an overview of the basic principles of the electrophysiology study.

**Keywords:** cardiac electrophysiology, electrophysiology, EP study, intracardiac electrogram, electrocardiography
