This text focuses on the practical, and theoretical when applicable, aspects of clinical electrophysiology of cardiac arrhythmias in the young. It represents a compilation of the clinical course, electrocardiograms, electrophysiologic studies, pharmacological management and transcatheter ablation therapy in patients from infancy through young adulthood seen at the clinical sites of the authors of this volume. The book is divided into two parts. The first part, Background (Chapters 1-3) discusses the cardiac conduction system - development, anatomy and physiology. Particular attention is directed to the clinical electrophysiology of the cardiac conduction system and the techniques of electrophysiologic study that are specific to children and that have been developed and practiced at the clinical centers represented in this volume. The second part, Cardiac Electrophysiology in Infants and Children focuses on the clinical science of cardiac arrhythmias in infants and children. Chapters 4-12 discuss the mechanisms, the ECG characteristics, electrophysiologic findings, treatment and prognosis of tachyarrhythmias. Chapters 13-16 focus on bradyarrhythmias. Chapters 17-20 address certain specialized subjects including syncope, cardiac pacemakers, implantable cardiac defibrillators, genetic disorders of the cardiac impulse, fetal arrhythmias and sudden cardiac death as it occurs in the young. Chapters 21-22 center on the pharmacology of antiarrhythmic agents, indications for use, doses, side effects and toxicity, as well as on transcatheter arrhythmia ablation. Finally, what the practitioner can expect to see from the impact of cardiac arrhythmias on the life of the patient and family is discussed from the nursing point of view in Chapter 23. This volume is intended for the young house officer or mature physician to increase their understanding, spark their interest and lead them to a therapeutic solution when faced with a child with a cardiac arrhythmia.