Social support is important to achieve beneficial changes in risk factors for disease, such as overweight and obesity. This paper presents the theoretical and practical framework for social support, and the mechanisms by which social support affects body weight. The theoretical and practical framework is supported with a literature review addressing studies involving a social support intervention for weight loss and weight loss maintenance. A major aspect in social support research and practice is the distinction between structural and functional support. Structural support refers to the availability of potential support-givers, while functional support refers to the perception of support. Interventions often affect structural support, for example, through peer groups, yet functional support shows a stronger correlation with health. Although positive correlations between social support and health have been shown, social support may also counteract health behaviour change. Most interventions discussed in this review showed positive health outcomes. Surprisingly, social support was clearly defined on a practical level in hardly any studies, and social support was assessed as an outcome variable in even fewer studies. Future social support intervention research would benefit from clear definitions of social support, a clear description of the intended mechanism of action and the actual intervention, and the inclusion of perceived social support as a study outcome. © 2005 Nature Publishing Group. All rights reserved.