Ventricular tachycardia (VT) is a life-threatening arrhythmia that is common to all forms of heart disease and an important cause of sudden death. Ventricular scars from infarction or replacement fibrosis provide a substrate for reentry that is a common cause. Understanding the pathophysiologic link between ventricular scars and ventricular tachycardia informs approaches to identify patients at risk, has led to development of methods to ablate the arrhythmia substrate that can be applied even in severe heart disease, and suggests future diagnostic and therapeutic strategies.