This article describes changes in the number of average weekly hours of market work per person in the United States since World War II. Overall, this number has been roughly constant; for various groups, however, it has shifted dramatically—from males to females, from older people to younger people, and from single- to married-person households. The article provides a unique look at how the lifetime pattern of work hours has changed since 1950 for different demographic groups. The article also documents several factors that may be related to the changes in hours worked: simultaneous changes in Social Security benefits, fertility rates, and family structure. The data presented are based on those collected by the U.S. Bureau of the Census during the 1950–90 decennial censuses.