The aim of this book is to be able to interpret Electrocardiograms avoiding all possible errors. The accuracy of the interpretation is of great importance but a true diagnosis is far more significant. This book focuses on the recognition and interpretation of arrhythmias, one of the most important clinical tools in medicine. The greatest degree of accuracy is achieved by familiarising with the normal ECG that enables the recognition of abnormal patterns to be made immediately. Firstly, it is necessary to acquaint the function of the heart and the electrical activity in order to broaden our understanding of how the ECG detects this electrical activity. It is essential to know the characteristic patterns of a normal ECG and to categorise a wide array of morphologic patterns along with determining abnormal ECG patterns to be diagnostic of particular pathological entities.