With a careful history, physical examination, and directed investigation, physicians can determine the likely cause of syncope in more than 50% and perhaps up to 80% of patients. Understanding the cause of syncope allows clinicians to determine the disposition of high- and low-risk patients. Patients with a potential malignant cause, such as a cardiac or neurologic condition, should be treated and admitted. Those with benign causes can be safely discharged. This article reviews the diagnosis and ED work-up of syncope, the different classifications of syncope, and prognosis. The use of specific decision rules in risk stratification and syncope in the pediatric population are discussed in another article.