Supraventricular tachycardia (SVT) is the most common symptomatic arrhythmias in children. Re-entry tachycardias are the most common form, on the contrary automatic tachycardias are relatively rare. There are four types or re-entry: along anomalous pathway with bi-directional (Wolff-Parkinson-White) or unidirectional conduction, intranodal re-entry, intra-atrial re-entry that is common after surgical procedure, and finally the uncommon sinus node re-entry. Automatic tachycardias may be atrial or junctional. The different types of tachycardia have a different incidence according to the age: in the first year of age re-entry along anomalous pathway is the dominant form, while intranodal reentry becomes common during adolescence. The age at the beginning of tachycardia is important for long term prognosis. When SVT starts in the first months of life it disappears in 80% of cases within the first year of life; on the contrary, if tachycardia starts later spontaneous remission is detected in only 15%-20% of patients. In infancy heart failure is the more common presenting symptom, thereafter palpitations become the principal cause of recognition of SVT. Syncope is reported in about 8% of cases and in another 15% usually neonates and infants, the SVT has an occasional detection. Electrocardiogram (ecg) usually allows the precise diagnosis of various types of SVT, and every effort should be made to record ecg during tachycardia. The parameters that should be evaluated are: heart rate, P wave axis, PR and RP interval, and finally presence or absence of AV block. Short lasting episodes should be difficult to be recorded; in these cases cardio-call and trans-telephonic transmission represent useful techniques to obtain SVT demonstration. Patients with SVT require a complete evaluation with others diagnostic techniques: echocardiogram, Holter monitoring, stress test, that should be chosen according the type of tachycardia. Electrophysiologic evaluation is now rarely performed for diagnostic purpose; trans-esophageal atrial stimulation being less invasive than intracardiac evaluation is more extensively employed when diagnosis of SVT is uncertain. Transesophageal stimulation is useful in the following situations: 1) evaluation of patients with symptoms suggestive of paroxistic tachycardia but without ecg documentation, 2) to assess the mechanism responsible for re-entry tachycardia: macro re-entry versus intranodal re-entry 3) to evaluate characteristics of anomalous pathway with bi-directional conduction, and 4)to terminate re-entrant SVT.