The Origins of Ventricular Arrhythmias: Using the ECG as a Key Tool for Localization is the third volume in a case-based series highlighting the importance of the 12-lead ECG when mapping and treating patients with ventricular arrhythmias.This volume features 30 cases, presented as a series of "unknowns" to teach electrophysiology and cardiology fellows-in-training the concept of connecting ventricular arrhythmias’ QRS morphology with the arrhythmia site of origin. These case studies are complemented by multimodality imaging and anatomy data to illustrate the precise locations of the sites of origin of different ventricular arrhythmias.Mapping approaches are discussed to emphasize how 12-lead ECGs help identify critical sites of the arrhythmias.Fully illustrated with more than 100 figures that include 12-lead ECGs, ECGs with intracardiac tracings, electroanatomic activation maps, and 3D reconstructions of echocardiographic images.These cases will benefit the reader by providing a reminder of the localizing value of ECGs in combination with appropriate imaging for the mapping and ablation of ventricular arrhythmias.