This chapter summarizes what is known about the labor supply of older American men, defi ned as those aged 55 years and over. The topic is of great interest because in the coming decades older individuals will comprise a much greater portion of the U.S. population, so the labor supply of older adults will have a signifi cant impact on national output, tax revenues, and the cost of means-tested programs. Most importantly, a greater proportion of older individuals will need to remain in the workforce than is the present case, because the retirement income system is contracting and working longer is the only way for most people to ensure fi nancial security in their old age. The paper's focus is on men, because women's work patterns are changing and increasingly refl ect the work patterns of men.Section I of this paper describes the changes to the U.S. retirement income system that will require people to work longer. Section II summarizes the long-term decline in labor force activity among older men over the course of the twentieth century, and the factors that contributed to this trend. Section III describes the recent turnaround in the labor force activity of older Americans, and the changes in Social Security and employer-provided pension plans that likely led to that reversal. In an attempt to determine whether the labor supply of older workers will continue to increase, section IV describes changes in work patterns that have emerged in the last 20 years, which have led to more labor market mobility and less job tenure among older workers, and the implications of such changes on labor supply. Section V addresses how the health of older people may infl uence the extent to which they can be expected to