There is now robust evidence of the protective effects of social relationships both on morbidity and on mortality, with a magnitude of effect comparable with many well-established risk factors such as smoking cessation and obesity. My program of research examines the influence of both the quantity and the quality of social relationships on long-term health and on risk for mortality, and the biological pathways (e.g., cardiovascular, neuroendocrine, genetic) by which such associations may occur. I also consider the potentially detrimental influence of negativity in close relationships (e.g., ambivalence, marital distress). My studies have examined social relationships at a network level, among married couples, in mother-and-infant relationships, and within friendships. My work is interdisciplinary and takes a multi-method approach including experimental, naturalistic, meta-analytic, and intervention studies.