The goal of this book is to teach the ability to form an autonomous and clinically useful opinion about any 12-lead electrocardiogram (EKG). It introduces basic principles of anatomy and physiology, including a review of the heart’s electrical system. The heart has an intricate electrical system, made up of highly specialized cells, that is responsible for generating each heart beat. The heart’s electrical system consists of five structures: the sinoatrial (SA node), the atrioventricular (AV node), the bundle of His, the right and left bundle branches, and the Purkinje fibers. One of the most basic yet important pieces of information the EKG provides is the heart rate (HR). The most accurate way to measure heart rate is by measuring the R-R interval. Learning the normal electrical direction of forces in the heart provides a simple and scientific way of understanding and interpreting an EKG. The book also discusses vital elements of cardiology, such as atrial and ventricular arrhythmias, conduction abnormalities and heart block, ischemic and nonischemic disorders, and more. It explains various types of heart blocks such as premature atrial contraction, sinus arrest and asystole, and various types of pacemakers such as ventricular pacemaker and artrial pacemakers. Drug effects and toxicities, electrolyte imbalances, trauma, pericardial diseases, lung disease, cancer, cardiomyopathies, and systemic diseases are conditions that can cause specific changes on the EKG.