The association between the quality of people’s close relationships and their physical health is well established. But from a psychological perspective, how do close relationships impact physical health? This article summarizes recent work seeking to identify the relationship processes and psychological mediators and moderators of the links between close relationships and health, with an emphasis on studies of married and cohabitating couples. We begin with a brief review of a recent meta-analysis of the links between marital quality and health. We then describe our strength and strain model of marriage and health, homing in on one process—partner responsiveness—and one moderator—adult attachment style—to illustrate ways in which basic relationship science can inform our understanding of how relationships impact physical health. We conclude with a brief discussion of promising directions in the study of close relationships and health.