Childhood obesity can cause social, psychological and health problems, and is linked to obesity later in life and poor health outcomes as an adult. Obesity development is related to physical activity and nutrition. To prevent obesity, 55 studies conducted internationally have looked at programmes aiming to improve either or both of these behaviours. Although many studies were able to improve childrens nutrition or physical activity to some extent, only some studies were able to see an effect of the programme on childrens levels of fatness. When we combined the studies, we were able to see that these programmes made a positive difference, but there was much variation between the study findings which we could not explain. Also, it appeared that the findings may be biased by missing small studies with negative findings.We also tried to work out why some programmes work better than others, and whether there was potential harm associated with children being involved in the programmes. Although only a few studies looked at whether programmes were harmful, the results suggest that those obesity prevention strategies do not increase body image concerns, unhealthy dieting practices, level of underweight, or unhealthy attitudes to weight, and that all children can benefit. It is important that more studies in very young children and adolescents are conducted to find out more about obesity prevention in these age groups, and also that we assess how long the intervention effects last. Also, we need to develop ways of ensuring that research findings benefit all children by embedding the successful programme activities into everyday practices in homes, schools, child care settings, the health system and the wider community.