Social relationships benefit not only mental health but also physical health. This review addresses the following questions: (1) What is the overall magnitude of the effect of social relationships on risk for premature death? (2) How generalized is the effect and are there factors known to influence this association? (3) What are the likely pathways by which social relationships influence longevity? The article concludes by discussing the implications of these findings for potential interventions aimed at reducing risk for mortality.