This meta–analysis studies the association between the child–staff ratio and the process quality (ERS, CLASS, CIS, ORCE, and EQOS) in center–based Early Childhood Education and Care (ECEC) institutions and identifies potential moderating factors. It synthesizes evidence from 49 publications from 14 countries with data from 18.049 ECEC classrooms. Results of the three–level random effects model provided a small significant negative correlation (Z = – 0.12, p<.001), indicating a negative relationship between an increase in the number of children per staff member on the process quality across publications. Additional moderator analyses showed that the correlation was higher for mixed–age classrooms and younger children and more relevant for global process quality indicators. Worse child-staff ratios correlated to a lesser degree with the interaction quality. Effect sizes also varied according to the type of staff included in the calculation of the child–staff ratio. These results underline that research evidence is rather heterogeneous and should not be interpreted without context. The authors discuss important implications for policy and pedagogical practice.