Background: The implementation of preventive interventions is considered a crucial aspect of their success. However, few studies have investigated which components of implementation are most important. Objective: We aimed to understand whether the components of implementation integrity--adherence, quality of delivery, dose, and participants' involvement--influenced the effectiveness of four parenting programs. We also investigated factors associated with these components. Method: Data come from a national evaluation of parenting programs in Sweden. The study was a randomised controlled effectiveness trial, with a sample of 535 parents with 3-12-year-old children. Measures included parenting behaviors (angry outbursts, harsh parenting, attempts to understand, rewarding, and praising), child conduct problems (ECBI and SNAP-V), and measures tapping into the four components (adherence, quality of delivery, dose, and participant involvement). Results: We ran multilevel models and found that implementation quality (adherence and quality of delivery) did not influence the effects on parents and children. Conversely, participant involvement was associated with improvements in parenting and child conduct. Finally, parents' perceptions of their leaders as supportive and understanding were associated with parents' responsiveness and attendance. Conclusions: Our study highlights the importance of having actively engaged parents to maximise intervention effects.