Hypertrophic cardiomyopathy (HCM) is regarded as the most common cause of sudden cardiac death in young people (including trained athletes). However, assessing sudden death (SD) risk and identifying the most appropriate candidates for prophylactic device therapy is a complex process compounded by the unpredictability of the underlying arrhythmogenic substrate, absence of a single dominant and quantitative risk maker for this heterogeneous disease, and also the difficulty encountered in assembling sufficiently powered prospective and randomized trials in large patient populations. Patients with multiple risk factors and most young patients with one strong and unequivocal risk marker can be considered candidates for primary prevention defibrillators. Despite certain limitations, the current risk factor algorithm (when combined with a measure of individual physician judgment) has proved to be an effective strategy for targeting high-risk status. This approach has served the HCM patient population well, as evidenced by the significant appropriate defibrillator intervention rates, although a very small proportion of patients without conventional risk factors may also be at risk for SD. Indeed, the introduction of implantable defibrillators to the HCM patient population represents a new paradigm for clinical practice, offering the only proven protection against SD by virtue of effectively terminating ventricular tachycardia/fibrillation. In the process, implantable defibrillators have altered the natural history of HCM, potentially providing the opportunity of normal or near-normal longevity for many patients. Prevention of SD is now an integral, albeit challenging, component of HCM management.