NSVT sometimes is an important marker of increased risk for subsequent tachyarrhythmias capable of causing syncope, cardiac arrest, or sudden cardiac death (SCD). It is not clear whether episodes of NSVT bear a cause-and-effect relationship with sustained tachyarrhythmias and SCD, or if they simply are a surrogate marker of cardiac dysfunction and electric instability. It is clear, however, that the prognostic significance of NSVT depends on the presence, type, and severity of underlying heart disease. To treat asymptomatic and symptomatic patients with NSVT properly, the clinician first must have a thorough understanding of the underlying cardiac substrate in a particular patient. The prognosis hinges on whether this is a patient with a structurally normal heart, coronary artery disease with prior myocardial infarction, a nonischemic dilated cardiomyopathy (NIDCM), or a hypertrophic cardiomyopathy (HCM). Knowing the anatomic substrate as well as overall left ventricular systolic function determines the prognostic significance of the NSVT, the potential role of programmed stimulation (electrophysiology study), and the appropriate management of the patient.