The worldwide prevalence of mental disorders in children is about 13 %. Disorders can be subdivided into internalizing and externalizing disorders, whereby both types of disorders go along with severe impairments. Regarding causes of psychiatric disorders, evidence for biological, psychological and environmental factors have been found. An early and effective treatment is important in order to change the course of disorders and to positively influence child development. The inclusion of parents in the treatment of children plays an important role for reaching this aim. The present cumulus introduces three publications that are dealing with the effectiveness of parent-based interventions in the treatment of internalizing and externalizing disorders of children. Since parents have a central role in child development, effects of parent trainings on specific parental variables will be presented as well. Publication 1 provides a systematic review of the current state of research on the effectiveness of parent-based interventions for the treatment of internalizing and externalizing disorders. Overall, there was evidence for effectiveness, whereby the research base on externalizing disorders was found to be broader than the one on internalizing disorders. However the available meta-analytic results on parent training effectiveness on externalizing behavior problems were found to be heterogeneous for specific parental characteristics as well as for child behavior. Therefore, in publication 2 and 3 previous findings were aggregated via meta-meta-analyses in order to obtain a more precise impression about the magnitude of effects on specific parental characteristics (parenting behavior, parental perceptions, parental mental health, parental relationship quality) and on child behavior (overall child behavior, externalizing behavior). Potential overlap of primary studies was taken into account in the calculations in order to counter possible bias. Regarding parental characteristics, moderate and stable effects were found for parenting behaviors and parental perceptions. However, the effect on parenting behavior was not supported by observational data. For parental mental health as well as for relationship quality, just a small effect was found, which was stable at follow-up only for relationship quality. Regarding parent training effectiveness on child behavior, moderate effects were shown on all variables (overall child behavior, externalizing behavior) across all measurement times (post-intervention, follow-up). Moreover effectiveness on child behavior was supported by observational data. The considerable heterogeneity that was observed throughout analyses will be addressed in the discussion with regard to possible causes. Overall, parent training interventions have proved to be an evidence-based intervention in the treatment of mental health disorders in children and should be applied in clinical practice.