Samenvatting

Evidence concerning psychosocial interventions for children and young people with externalizing behavior problems has amassed at an impressive pace in recent years. Interventions that have been proven effective are now considered vehicles through which the knowledge of “what works” can be applied in practice. Outcomes for children, young people, and their families, however, have not improved in line with these advances in knowledge. This difference between the knowledge of “what works” and the application of this knowledge in real-life practice has become known as the “implementation gap”. This dissertation explores questions considering the implementation gap, with a focus on whether professionals are delivering the interventions as intended (treatment integrity). The results of the research underlying this dissertation show that 1) although measuring treatment integrity is important, it is often missing or not examined under adequate circumstances in studies, 2) applying interventions with a high level of treatment integrity makes a real difference to the end-users of the services and 3) targeted and continued support to professionals with a focus on providing feedback on levels of treatment integrity is necessary to enable them to deliver interventions as intended. Organizing support around common factors of interventions can be a first step in integrating and providing feasible support for professionals that provide more than one intervention.