Social networks reflect the structure of our interpersonal relationships. The effect of social networks on health is a topic of growing interest, particularly in an increasingly connected world. This review provides an overview of how social relations shape obesity risk and the effectiveness of network-based obesity interventions across the lifecourse. The review highlights that while the literature suggests obesity and related health behaviors are similar between socially connected individuals, why this is the case and how to effectively intervene remain unclear. In addition, the review outlines methodological gaps limiting our understanding of how social networks shape obesity risk throughout the lifecourse. Several implications for obesity prevention and research are offered, including the need to examine the relationship of social networks and obesity across rather than within lifecourse stages, continued development of statistical social network analysis methods, and the need for new cohort studies, particularly among children and the elderly.