An increasing number of preschool children are becoming overweight. Although many risk factors have been identified for school-age children, less is known about this young age group. Ecological models have been developed to illustrate how individual characteristics, family characteristics, community-level factors, and policies may influence weight gain. We used this model to review factors that influence overweight in children, aged six months to five years, which are amenable to policy intervention in resource-rich countries. We found strong evidence for a direct association between childhood overweight and maternal prepregnancy body size, maternal smoking during pregnancy, and children's television/media use; strong evidence for an inverse relationship between breastfeeding and overweight, and moderate evidence for children's physical activity. There was limited research on community-level factors, policies and interventions. Future policies and interventions should be subject to evaluation and aim to support parents and young children to develop health-related behaviours that may prevent early childhood overweight.