As the U.S. population ages, traditional sources of retirement income will likely fall short of what is needed to maintain pre-retirement living standards for many individuals. The issue of retirement security is especially important for women, because even today nearly 30 percent of single women, who represent a majority of households at older ages, are classified as poor or near-poor. One solution to the retirement security challenge is for women to work more during their lifetimes and to stay in the workforce longer as they age. By and large, those who continue to work until their mid-60s or beyond do not end up poor. The question is explored in this study is what determines women’s labor force activity at older ages and what determines when they retire. Only by understanding these levers is it possible to make changes that are likely to encourage stronger labor force participation, and thus greater retirement security, for women.