Research Findings: Early childhood mental health consultation aims to reduce problem behaviors and improve social skills in young children primarily through changes in the classroom environment and teacher practices. We conducted a systematic review of the literature and identified 14 rigorous studies that reported on child-level outcomes. These studies had at least one of the following characteristics: publication in a peer-reviewed journal, use of a randomized control trial design, or inclusion of a comparison group. Across these studies, there was variation in the approaches to consultation, qualifications of the consultants, and intensity of the services provided. Overall, early childhood mental health consultation services were consistently associated with reductions in teacher-reported externalizing behaviors. Findings related to reductions in internalizing behaviors were mixed. Teacher ratings of prosocial behaviors were improved in the majority of the studies that reported on this domain. Practice or Policy: This research synthesis underscores the importance of documenting the specific approaches to early childhood mental health consultation that are being implemented to allow for replication of effective models. Areas for future research improvement include increasing rigor through studies that contain independent assessments of children's behaviors, isolating key components of effective consultation, and identifying consultant qualifications and characteristics that lead to child behavior changes. (Contains 2 tables.)