Short summary: This study reviews the European evidence on the impact of professional development (PD) of pre-school educators on child outcomes. A meta-analysis is used to investigate how PD of pre-school educators affects child outcomes. The studies included in the meta-analysis all concern PD in formal pre-school centers in Europe, and focus is on effects directly on child outcomes. Thus, potential effects of PD on the pre-school educators' skills, beliefs, attitudes and work conditions are not investigated. The geographical restriction excludes the wide range of studies from the US, but these studies still form the base for comparison of the estimated effect sizes. The range of relevant European studies is quite small, but despite the small number of studies included, our results show a significantly positive effect of PD on child outcomes. We find an overall effect size of 0.35 (with a 95 % confidence interval from 0.20 to 0.51).