Sudden unexpected death, often occurring in young, asymptomatic patients, is the most devastating facet of the natural history of hypertrophic cardiomyopathy, and appears to be the consequence of primary ventricular tachyarrhythmias arising in an electrically unstable myocardial substrate characterized by disorganized cellular architecture, ischemia, cell death, and replacement scarring. Although precise identification of all hypertrophic cardiomyopathy patients at high risk for a catastrophic event is a clinical challenge, effective treatment strategies for the prevention of sudden death with the implantable cardioverter-defibrillator are now available.