It is well-recognized that close relationships influence emotional well-being and psychological outcomes, however often overlooked is the influence close relationships have on physical health. This chapter summarizes the evidence supporting taking close relationships seriously for one’s physical health. Epidemiological evidence linking close relationships to risk for premature mortality and experimental evidence in animals, together point to a directional and causal effect on health. Major theoretical approaches (Attachment, Social Network, Social Support), general models (Direct effects, Social Control/Influence), stress-based models (Stress buffering, Matching hypothesis/Responsiveness, Allostasis) and models of relationship thriving (Social Capitalization) further explain why such an association may exist. Importantly, evidence has accumulated documenting behavioral and biological pathways. Close relationships influence a variety of behavioral processes, including health behaviors, medical adherence/compliance, sleep, physical touch/affection—all of which influence physical health. There is also substantial evidence supporting the influence of close relationships on biological pathways, including cardiovascular, neuroendocrine, and immune functioning, that can have direct influences on health. The quality of close relationships may moderate such associations and therefore further attention to relationship quality in future research. Finally, future research is needed examining the role and implications of changing technology on the broader associations between close relationships and physical health.