Atrial flutter and fibrillation are infrequently encountered in the young patient without congenital heart disease. Lone atrial flutter appears to be more common in boys and is most often present at birth. Cases diagnosed beyond the first week of life may present with tachycardia-induced cardiomyopathy, which typically resolves upon restoration of sinus rhythm. While antiarrhythmic drug therapy and catheter ablation may be indicated in some, most patients experience no recurrence on follow-up. Lone atrial fibrillation, though equally rare in children, typically presents in adolescence. In the absence of structural heart disease, atrial fibrillation in the young most frequently occurs in patients with Wolff–Parkinson–White syndrome or channelopathies, such as long and short QT and Brugada syndromes. Atrial fibrillation may also be triggered by acquired or iatrogenic conditions that result in atrial remodelling, inflammation, infiltration, and/or autonomic or neuro-hormonal imbalances. In the absence of pediatric evidence-based guidelines, management decisions regarding antiarrhythmic and anticoagulation therapy are largely inferred and adapted from the adult literature. Many questions remain unanswered, including thromboembolic risk estimates, pathophysiological mechanisms, potential associations with environmental factors such as competitive sports, and underlying genetic determinants.