This article reviews recent medical and social science literature on sexual functioning in older adults. We provide a broad definition of sexual functioning that includes a range of solo and partnered forms of sexual expression. We identify four determinants of sexual functioning: biologic, psychological, social context (including culture), and interactions of these with each other. Recent literature on the impact of aging and physical health documents some decline in frequency of sexual activity. Interest continues in the role of hormones in male and female sexual functioning. Recent research highlights the role of the social context, especially the presence of a sexual partner and the relationship with that partner, in sexual activity. We discuss variations in sexual functioning by life course events, gender, and race and ethnicity. Relevant results from the Global Study of Sexual Attitudes and Behaviors; Male Attitudes Regarding Sexual Health Survey; and the National Social Life, Health and Aging Project are also reviewed.