This Population Bulletin chronicles changes in world population in the last century, with a particular focus on the last 50 years. It examines the social and economic factors that affect population change, including wide disparities in income, education, and women’s status within countries. It also discusses the heightened international concern since the 1950s about rapid population growth, widespread fertility declines, and the new world consensus reached in the 1990s about how best to respond to population trends. It reviews the factors that have led to low fertility in Europe, Japan, and other areas and how governments there have begun to respond. Whether or not these responses bring the desired population change, their common goal is to improve the quality of life for individuals in the 21st century.