Background: In contrast to the adult population with congenital heart disease (CHD), arrhythmia mechanisms and outcomes of ablation in pediatric patients with CHD in recent era have not been studied in detail. Aims of this study were to determine arrhythmia mechanisms and to evaluate procedural and long-term outcomes in pediatric patients with CHD undergoing catheter ablation. Methods: Consecutive patients <18 years of age with CHD undergoing catheter ablation over an 11-year period (2007–2018) were included. Procedural outcome included complete or partial success, failure or empirical ablation. Long-term outcome included arrhythmia recurrence and burden according to a 12-point clinical arrhythmia severity score. Results: The study population consisted of 232 patients (11.7 years [0.01–17.8], 33.5 kg [2.2–130.1]). The most common diagnoses were Ebstein’s anomaly (n=44), septal defects (n=39), and single ventricle (n=36). Arrhythmia mechanisms included atrioventricular reentry tachycardia (n=104, 90 patients), atrioventricular nodal reentry tachycardia (n=33, 29 patients), twin atrioventricular nodal tachycardia (n=3, 2 patients), macroreentrant atrial tachycardia (n=59, 56 patients), focal atrial tachycardia (n=33, 25 patients), ventricular ectopy (n=10, 8 patients), and ventricular tachycardia (n=15, 13 patients). Fifty-six arrhythmias (39 patients) were undefined. Outcomes included complete success (n=189, 81%), partial success (n=7, 3%), failure (n=16, 7%), or empirical ablation (n=20, 9%). Over 3.6 years (0.3–10.7) arrhythmia recurred in 49%. Independent of arrhythmia recurrence, arrhythmia scores decreased from 4 (0–10) at baseline to 0.5 (0–8) at 4 years follow-up ( P <0.001). In 23/51 repeat procedures (45%), a different arrhythmia substrate was found. Overall adverse event rate was 9.4%, although only 1.6% (n=4) were of major severity and 0.8% (n=2) of moderate severity. Conclusions: Pediatric patients with CHD demonstrate a broad spectrum of arrhythmia mechanisms. Despite recurrence and emergence of novel mechanisms after a successful procedure, ablation can be performed safely and successfully resulting in decreased arrhythmia burden.