Arrhythmias in neonates and infants require a specific management due to the particular nature of the rhythm anomalies in children. Accurate diagnosis of the tachycardia is realised mainly by means of ECG recording and vagal manoeuvres. The nature of the tachycardia will determine management, therapy and prognosis. In neonates <3 months, supraventricular tachycardia due to Wolff-Parkinson-White syndrome represents 70% of all supraventricular tachycardias, which, after conversion by vagal manoeuvres, requires a preventive treatment by digoxine (10 y/kg/day tid). Neonatal flutter occurs in infants without structural heart disease. It has an excellent prognosis after conversion to sinus rhythm by transoesophageal pacing. Atrial tachycardia is less frequent but can induce a tachymyocardiopathy and often requires combined therapy including amiodarone. Long QT syndrome, clinically and genetically heterogeneous, is characterized by a prolongation of the QT interval (QTc > 440 ms) and a high risk of syncope and sudden death due to malignant ventricular arrhythmias. Beta-blockers significantly decrease cardiac events during follow-up. Congenital atrio-ventricular block is rare but potentially lethal in the first months of life in the absence of permanent pacing. The morbidity remains high during long term follow-up in unpaced children.