There is a strong belief that individuals are particularly vulnerable to social isolation and loneliness as they age because of predictable events associated with aging such as retirement, widowhood, and declining health. However, there is a theoretical and empirical consensus that social isolation is not an inevitable part of aging. Research on the life course using longitudinal designs has identified risk factors for social isolation and loneliness and their negative consequences. This chapter reviews recent population trends that may increase social isolation and loneliness, discusses how to measure these states, summarizes findings on their prevalence among older people, highlights important new findings from the literature, and comments on future directions in research on social isolation, loneliness and health, including important knowledge gaps that need to be addressed.