In developed countries, men's labor force participation at older ages has increased in recent years, reversing a decades-long pattern of decline. Participation rates for older women have also been rising. What explains these patterns, and the differences in them across countries? The answers to these questions are pivotal as countries face fiscal and retirement security challenges posed by longer life-spans. This eighth phase of the International Social Security project, which compares the social security and retirement experiences of twelve developed countries, documents trends in participation and employment and explores reasons for the rising participation rates of older workers. The chapters use a common template for analysis, which facilitates comparison of results across countries. Using within-country natural experiments and cross-country comparisons, the researchers study the impact of improving health and education, changes in the occupation mix, the retirement incentives of social security programs, and the emergence of women in the workplace, on labor markets. The findings suggest that social security reforms and other factors such as the movement of women into the labor force have played an important role in labor force participation trends.