The isolated syncopal episode, most commonly a benign and self-limited event, may also represent serious neurologic or cardiologic disease. In evaluating patients with syncope, the clinician must differentiate the benign from the more serious etiology. Considerable progress has been made in the past 20 years in understanding the causes of syncope and in determining prognosis. This paper reviews our current appreciation of syncope and in determining prognosis. This paper reviews our current appreciation of these issues and defines the clinical utility of diagnostic testing in patients with syncope.