Our purpose in writing this book was to produce a clinically-oriented, n-multi-au- thored textbook ofcardiac electrophysiology that would be useful to practicing electro- physiologists, cardiologists, fellows in training as well as associated electrophysiology professionals, including nurses and techlogists. While all clinical textbooks risk be- coming outdated even before they're published, and few textbooks ofa manageable size can claim to be completely comprehensive, our goal was to produce a book that system- atically presents a thorough discussion ofthe fundamental principles and concepts im- portant to the practice of clinical electrophysiology. We do t discuss basic cellular electrophysiology for its sake alone, but instead include basic science material only when itis helpful in explaining the overlying clinical principles. Cardiac electrophysiology, as with any subspecialty, behaves as a living organism with continuous evolution of its standards and practices. However, even though the details and tools of management (catheters, drugs, devices, etc. ) may change with dazzling speed, the fundamental princi- ples ofdiagsis and management generally change very little and they remain the criti- cal underpinning ofthe day-to-day management ofpatients with cardiac arrhythmias. In the first third ofthe book we present the principles ofclinical cardiac electrophysiolo- gy as it is currently practiced.