PROJECT SUMMARY Loneliness is a known risk factor for physical decline and mortality among older adults. Further, loneliness is distinct from social isolation and has its own unique effects on health. Yet little research has examined the potential dyadic impacts of loneliness for health among married older couples. Although marital status can protect against loneliness, this benefit is contingent upon marital quality. Moreover, loneliness can be “contagious” within older adults’ marriages, spreading from one partner to the other. Thus, married older adults in lower-quality marriages – or who have lonely partners – are at heightened risk of experiencing loneliness themselves. Therefore, loneliness may lead to downstream harms to health for both partners in a marriage. Potential mechanisms for such effects include: (a) stress, anxiety, or other negative health- harming experiences that result from having a lonely partner; (b) from the contagion of loneliness itself within marriage, and its consequent harms to health for each individual dyad member; and (c) from the contagion of deleterious health behaviors that result from experiencing loneliness. Using multi-wave longitudinal dyadic data from the 2006-2016 waves of the Health and Retirement Study, the proposed research will explore each of these potential pathways in turn, in order to demonstrate not only whether but also how loneliness gets “under the skin” to impair older adults’ health in a dyadic context. Findings will inform future research, policy, and practice concerning the reduction of loneliness and promotion of healthy aging among the older population.