Hospital readmission after pediatric cardiac surgery is incompletely understood. This study analyzed data from The Society of Thoracic Surgeons Congenital Heart Surgery Database to determine prevalence, to describe patient characteristics, and to evaluate risk factors for readmission. Readmission was defined by the "readmission within 30 days after discharge" field. Routine variables were summarized. Regression analysis was used to identify factors associated with readmission. The study cohort included 56,429 patient records from 100 centers. Overall, 6,208 (11%) patients were readmitted. The most common reasons for readmission were respiratory or airway complications (14.2%), septic or infectious complications (11.4%), and reasons not related to the preceding surgical procedure (20.2%). Primary reason for readmission varied across benchmark operation groups. In multivariable analysis, factors associated with increased odds of readmission included the presence of noncardiac abnormalities (odds ratio [OR], 1.24), chromosomal abnormalities or genetic syndromes (OR, 1.24), preoperative mechanical circulatory support (OR, 1.36), other preoperative factors (OR,1.21), prior cardiac surgery (OR, 1.31), Hispanic ethnicity (OR, 1.13), higher STAT procedural complexity (Society of Thoracic Surgeons/European Association for Cardio-Thoracic Surgery) (STAT level 3 vs 1, OR, 1.22; STAT 4 vs 1, OR, 1.48; STAT 5 vs 1, OR, 2.62), prolonged postoperative length of stay (OR, 1.07 per day from 0 to 14 days; OR, 1.01 per week >14 days), any major complication (OR, 1.27), any other postoperative complications(OR, 2.00), and discharge on a weekday (OR, 1.07). Readmission is common after congenital heart surgery, mostly for noncardiovascular reasons. Process improvement initiatives targeted at high-risk patients could minimize its impact.