Background: Parenting programs aim to reduce children’s conduct problems through improvement of family dynamics. To date, research on the precise benefits and possible harms of parenting programs on family well-being has been unsystematic and likely to be subject to selective outcome reporting and publication bias. Better understanding of program benefits and harms requires full disclosure by researchers of all included measures, and large enough numbers of participants to be able to detect small effects and estimate them precisely. Methods: We obtained individual participant data for 14 of 15 randomized controlled trials on the Incredible Years parenting program in Europe (total N = 1,799). We used multilevel modeling to estimate program effects on 13 parent-reported outcomes, including parenting practices, children’s mental health, and parental mental health. Results: Parental use of praise, corporal punishment, threats, and shouting improved, while parental use of tangible rewards, monitoring, or laxness did not. Children’s conduct problems and attention deficit hyperactivity disorder (ADHD) symptoms improved, while emotional problems did not. Parental mental health (depressive symptoms, self-efficacy, and stress) did not improve. There was no evidence of harmful effects. Conclusions: The Incredible Years parenting program improves the aspects of family well-being that it is primarily designed to improve: parenting and children’s conduct problems. It also improves parent-reported ADHD symptoms in children. Wider benefits are limited: the program does not improve children’s emotional problems or parental mental health. There are no signs of harm on any of the target outcomes.