Sudden cardiac death (SCD) is the most common cause of cardiovascular death worldwide, accounting for approximately 300,000 deaths in the U.S. annually, although estimates have ranged from 200,000 to 450,000 deaths. Operationally, SCD is most frequently defined as a cardiac death that occurred within 1 hour of cardiac symptom onset and without another probable cause of death. Studies from epidemiological cohorts from the 1970s through the 1990s suggest that 88 to 91% of deaths that occur within 1 hour of symptom onset are arrhythmic in nature. The temporal definition of SCD strongly influences epidemiological data. Increasing the time window to 24 hour since symptom onset to define SCD increases the sensitivity but reduces specificity by reducing the proportion of all sudden natural deaths that are due to cardiac causes. Approximately three-quarters of cases of SCD are caused by ventricular tachyarrhythmias such as ventricular tachycardia and ventricular fibrillation. Sustained ventricular arrhythmias may lead to hemodynamic instability and abrupt loss of consciousness without spontaneous recovery, requiring cardiac resuscitation (i.e., cardiac arrest). Prevention is the primary strategy to lower death from SCD. However, SCD is a particular management challenge because the majority of cases occur in individuals without a prior diagnosis of cardiac disease or other clear risk factors for SCD. The most common underlying cardiovascular diagnosis among people with SCD is coronary artery disease (CAD). Yet, in about half of the cases of SCD, SCD itself is the initial manifestation of CAD. The clinical strategy to prevent death from SCD involves identification of risk factors for ventricular tachyarrhythmias and SCD, to target individuals for medical and interventional treatments. This Technology Assessment examines the state of evidence related to ICD use for primary prevention of SCD. It examines the effectiveness of treatment with an ICD versus control treatment without an ICD. It also examines the effectiveness of combining an ICD with ATP or with CRT versus an ICD alone. This Technology Assessment considers evidence regarding the following three Key Questions: Key Question 1 a) In candidates for ICD implantation for primary prevention of SCD, what are the effects of ICD compared with no ICD therapy on clinical outcomes and patient-reported outcomes? b) In candidates for ICD implantation for primary prevention of SCD, what are the effects of ICD with ATP versus ICD alone, or of ICD with CRT versus ICD alone on clinical outcomes and patient-reported outcomes? Key Question 2 a) What are the adverse events related to treatment with an ICD for primary prevention of SCD? Specifically: i. Early (during hospitalization for implantation) ii. Late iii. Inappropriate shocks b) How do adverse events vary within the following subgroups? i. Different patient characteristics such as varying demographic features and major comorbidities ii. Different ICD characteristics iii. Different characteristics of clinicians implanting ICDs-that is, different levels of training and experience iv. Different characteristics of facilities where ICDs are implanted Key Question 3 Which patients have been included in comparative studies of ICDs for primary prevention of SCD? a) What were eligibility criteria for patients in studies included for Key Question 1? How were patients evaluated and what diagnostic tests and algorithms were used to select patients? b) Among patients in studies included for Key Question 1, what was the likelihood of SCD or ventricular tachyarrhythmia, as measured by total shocks for those with ICDs or episodes of SCD for those without ICDs?