BACKGROUND: This review synthesises evidence regarding associations between young people's social networks and their eating behaviours/bodyweight, and also explores how these vary according to the setting and sample characteristics. METHODS: A systematic review of cross-sectional and longitudinal observational studies examining the association between measures of young people's social networks based on sociometric data and eating behaviours (including calorific intake) and/or bodyweight. RESULTS: There is consistent evidence that school friends are significantly similar in terms of their body mass index, and friends with the highest body mass index appear to be most similar. Overweight youth are also less likely to be popular and more likely to be socially isolated at school. Frequency of fast food consumption has also been found to cluster within groups of boys, as have body image concerns, dieting and eating disorders among girls. CONCLUSION: School friendships may be critical in shaping young people's eating behaviours and bodyweight and/or vice versa, and suggests the potential of social-network-based health promotion interventions in schools. Further longitudinal research is needed to examine the processes via which this clustering occurs, how it varies according to school context, and the effects of non-school networks.