The following article in this series will describe common arrhythmias seen in the pediatric population. Their definitions and clinical presentations along with electrocardiogram (ECG) examples will be presented. In addition, ECG changes seen in acute toxic ingestions commonly seen in children will be described, even if such ingestions do not produce arrhythmias per se. Disturbances of rhythm seen frequently in patients with unrepaired and corrected congenital heart disease will be more fully discussed in the third article of this series. Numerous classification schemes for arrhythmias exist; in this article arrhythmias will be grouped based upon their major ECG manifestations.