During the past decade, the mechanisms of SVT have been discovered by intracardiac electrophysiologic studies. The differentiation of the various forms of tachycardia allows a more rational approach to therapy. Bedside maneuvers and the 12-lead ECG will identify the correct mechanism of SVT in the majority of patients. Diagnostic clues from the surface ECG include the position and morphology of the P wave, heart rate, change in rate with bundle-branch block, presence of AV block, and the initiation and termination of the SVT. With the use of bedside maneuvers and clues from the surface ECG, an intracardiac electrophysiologic study would be indicated only for recurrent SVT that is refractory to medical therapy and is potentially life-threatening.