Worldwide, a large number of infants, toddlers, and preschoolers are enroled in formal non-parental early childhood education or care (ECEC). Theoretically, lower adult/child ratios (fewer children per adult) and smaller group sizes are hypothesised to be associated with positive child outcomes in ECEC. A lower adult/child ratio and a smaller group size may increase both the extent and quality of adult/child interactions during the day. This systematic review examines the effects of reducing adult/child ratios and group sizes on process quality and on individual children's psychosocial adjustment, development and well-being in ECEC for children aged 0-5 years old. The review analyses evidence from 12 studies, two of which were randomised control trials, representing eight different populations. The main finding of the present review is that there are surprisingly few quantitative studies exploring the effects of changes to adult/child ratio and group size in ECEC on measures of process quality and on child outcomes. The review should be interpreted cautiously due to the low quality of the evidence. However, results tentatively support the theoretical impact of two structural features--adult/child ratio and group size--on process quality, which is reflected in legislation and quality standards imposing minimum requirements on the adult/child ratio and a maximum group size in ECEC settings. Findings from the review serve as a testimony to the urgent need for more contemporary research on the effects of adult/child ratio and group size in ECEC.