The vast majority of patients dying suddenly within 1 h after the onset of their symptoms have cardiac arrest in the setting of structural heart disease. Such events are most frequently due to ventricular tachyarrhythmias, such as rapid ventricular tachycardia (VT) and ventricular fibrillation (VF). In a minority of patients these arrhythmias occur in the apparently normal heart. A patient with an “apparently normal” heart can fall in either of two categories: (1) patients with no evidence at all of structural heart disease and (2) those with a minor disorder and/or not easy detectable structural disorder (Table 1).