Cardiac dysrhythmias in children can be due to a structural abnormality or an intrinsic defect in the electrical conduction system of the heart. In a child with a structurally normal heart, the mechanisms for dysrhythmias are the same as in adults, although the incidence and prevalence are different. Supraventricular tachycardias (SVTs) originate above the ventricles. In children, the two peak ages of onset for SVT are from before birth through the first year of life, and from ages 6 to 8 years. In most cases, there is spontaneous clinical resolution within the first year. When treatment is necessary, it includes vagal maneuvers, pharmacotherapy, and ablation. Wolff-Parkinson-White syndrome is a type of reentrant SVT that involves an accessory pathway. Long QT syndrome is a prolongation of the QT interval due to a genetic channelopathy. Bradycardia and heart block can be due to maternal autoimmune antibodies or structural heart defects. Ventricular tachyarrhythmias can be due to underlying structural heart disease, cardiomyopathy, or a metabolic derangement and may lead to sudden cardiac death.