The present review systematically explored research examining the relationship between therapist-related factors and the outcomes of parent interventions directed at children's behavior problems. A systematic search of the literature was conducted with online scientific databases, parenting programs, web sites, and bibliographic references of the selected articles, according to PRISMA guidelines. A total of 24 quantitative studies met the inclusion criteria. Although some methodological limitations were identified with respect to the measurement of therapist factors, the reviewed research strongly suggests that the therapist plays a critical role in parent interventions directed at behavior problems. In particular, many parent outcomes are found to be related to the parent-therapist alliance, the therapist's fidelity to the intervention, specific therapist's in-session actions, and the therapist's personal variables. The parent-therapist alliance and therapist fidelity to the intervention consistently relate to changes in parenting practices, and alliance additionally relates to fewer perceived barriers to participation in treatment, more treatment acceptability, and greater parenting satisfaction and self-efficacy. In addition, specific in-session therapist interpersonal actions relate to parents' engagement and satisfaction, while both the therapist's interpersonal actions and more active skills relate to parent change. Therapist's personal variables have been scarcely or poorly studied to date, but the results found justify the need to develop further research in this area. In conclusion, more attention should be given to the role of the therapist when implementing parenting programs directed at behavior problems, and more and better research is needed that can overcome the methodological limitations identified.