Social isolation (the objective state of having few social relationships or infrequent social contact with others) and loneliness (a subjective feeling of being isolated) are public health risks that affect a significant portion of the older adult population. Strong evidence suggests that many older adults are socially isolated or lonely in ways that puts their health at risk. At the request of the AARP Foundation, this consensus study will examine how social isolation and loneliness affect health and quality of life in adults aged 50 and older, particularly among low-income, underserved, and vulnerable subpopulations (or those considered “at risk”).