Hypertrophic cardiomyopathy (HCM) is the most common cause of sudden cardiac death (SCD) in young people, including trained athletes. It is now 30 years since the introduction of implantable cardioverter-defibrillators (ICDs) to clinical cardiovascular practice and coronary artery disease, and now device therapy represents the most significant therapeutic innovation and the only definitive strategy for prolonging the life of HCM patients. ICDs have proved effective in preventing SCD in young HCM patients with appropriate intervention rates of 11% for secondary and 4% for primary prevention, despite massive left ventricular (LV) hypertrophy, LV outflow obstruction, diastolic dysfunction or microvascular ischemia. Targeting candidates for prophylactic ICD therapy can be complex, compounded by the unpredictability of the arrhythmogenic substrate, the absence of a dominant risk factor, and difficulty in assembling randomized trials. However, a single major risk factor is often sufficient to justify an ICD, although additional markers and other disease features can resolve ambiguous decision-making. Nevertheless, the absence of all risk factors does not convey absolute immunity to SCD. The current risk factor algorithm, when combined with a measure of individual physician judgment (and patient autonomy considerations), is an effective guide to identifying high-risk HCM patients. ICDs have altered the natural history of HCM for many patients and provided an opportunity to achieve many decades of productive life, and the potential for normal or near-normal longevity. Indeed, prevention of SCD has now become a new paradigm in the management of HCM. (Circ J 2010; 74: 2271-2282)