•We meta-analyzed PD that focused on school readiness, social/emotional functioning through interactions, and language and literacy.•Most analyses revealed small positive effects of PD programs on child outcomes.•Stronger associations were found for outcomes related to the focus of the PD program.•Systematic review revealed more associations for longer programs with coaching.•Findings suggest that PD helps to shape children's learning. Professional development (PD) appears to be an important lever for improving practice in Early Childhood Education and Care (ECEC) programs. However, the availability of a large number of PD programs targeting different content areas makes it difficult to know where to invest limited resources. The primary goal of this review is to integrate the evidence on associations between PD programs targeting different content areas and outcomes that are conceptually linked to those content areas among preschool children. Where possible, PD mode of delivery, dose, study design, and author effects were also reviewed. Three electronic databases (Medline, PsycINFO, ERIC), websites of large datasets, and reference sections of all included articles, were searched up to February 5, 2018. Sixty-four studies of 30,209 preschool-aged children from 3327 classrooms in 1568 centers were included in the systematic review and 20 were included in meta-analyses. Over 400 unique child outcome measures were used. We conducted 13 meta-analyses on PD programs that targeted: School Readiness, Social/Emotional Functioning through Interactions, and Language/Literacy. Ten of the 13 meta-analyses were significant (effect sizes ranged from 0.07–0.26). Positive associations were identified when child outcomes aligned with the content of PD programs (e.g., Language/Literacy PD and identifying letters). The systematic review similarly identified more associations when outcomes were related to PD content and for programs that included a coaching component, were shorter, and used author-created outcome measures. Limitations, future research, and policy implications are discussed.