Approximately 70% of cases of sudden cardiac death are attributable to underlying CHD; in up to 40% of patients, sudden cardiac death may be the initial manifestation of CHD. The initiating arrhythmia in most patients is unknown but is presumed to be sustained monomorphic ventricular tachycardia, polymorphic ventricular tachycardia, or primary ventricular fibrillation (especially in the setting of acute ischemia). Complete heart block and sinus node arrest may also cause sudden death. A disproportionate number of sudden deaths occur in the early morning hours, which suggests that there is a strong interplay with the autonomic nervous system. Other forms of structural heart disease can predispose to sudden cardiac death including idiopathic cardiomyopathy, hypertrophic cardiomyopathy, valvular heart disease (aortic stenosis, pulmonic stenosis), congenital heart disease, arrhythmogenic RV cardiomyopathy, and myocarditis. Five to 10% of cases of sudden cardiac death are primarily arrhythmic and occur in the absence of structural heart disease. Etiologies include long QT syndrome, Brugada syndrome, and catecholaminergic polymorphic ventricular tachycardia. Prompt evaluation to exclude reversible causes of sudden cardiac arrest should begin immediately following resuscitation. Laboratory testing should be performed to exclude severe electrolyte abnormalities (particularly hypokalemia and hypomagnesemia) and acidosis and to evaluate cardiac biomarkers. Caution should be taken in attributing cardiac arrest solely to an electrolyte disturbance, however, because laboratory abnormalities may be secondary to resuscitation and not causative of the event. A 12-lead ECG should be performed to evaluate for ongoing ischemia or conduction system disease. Ventricular function should be evaluated with echocardiography. Coronary arteriography should be performed to exclude coronary disease as the underlying cause, since revascularization may prevent recurrence. In the absence of coronary disease, contrast-enhanced cardiac MRI may be used to evaluate for the presence of myocardial scar, which is a strong predictor of recurrent ventricular tachycardia/ventricular fibrillation in patients with nonischemic cardiomyopathy.