The aim of this dissertation is to better understand how the quality of social experience is related to allostatic load (AL), an index of cumulative dysregulation across physiological systems. More specifically, the project examines the effects of support and negativity from several sources on AL in a large community sample of middle-aged and older adults from the Midlife in the United States (MIDUS) study (N = 949, age 34-84). Results indicated that higher levels of negativity across the social network and lower levels of support from a spouse were each associated with higher AL, controlling for age and relevant demographic and health covariates. There was evidence that the effects of both network support and negativity varied by age, such that the association between social relationship quality and AL became weaker with age. This study is the first to demonstrate associations between social relationships and AL in a sample of adults representing five decades of adulthood, rather than in a limited range of older adulthood. In addition, this study extends previous work by testing for independent effects of support and negativity from specific sources. Taken together, these findings provide support for theoretical formulations arguing that AL is a pathway linking the quality of social relationships with morbidity and mortality.