Electrophysiologic testing represents a major advance in both the diagnosis and management of cardiac rhythm and conduction disturbances. It is a sophisticated but arduous and invasive technique that requires proper patient selection, trained personnel, and specialized equipment and facilities for its optimal application. Patients with serious or life-threatening rhythm disorders are most likely to benefit from electrophysiologic testing. Patients with cardiovascular disease and incapacitating symptoms such as syncope or near-syncope, for whom noninvasive testing fails to reveal an etiology, as well as those patients considered candidates for various pacemaker and antiarrhythmic surgical interventions, are also suitable candidates for electrophysiologic testing. This relatively young field is still rapidly evolving. The major advance to come in the field of electrophysiology will be the simple identification of patients at high risk for sudden cardiac death. It is unclear whether invasive electrophysiologic testing will ultimately fulfill this goal or whether it will be helpful instead in leading to the development of noninvasive methods which may satisfy this need. This symposium and the introductory article address the present status of electrophysiologic studies in clinical practice.