Through the first half of the 20th century, the study of human development was largely devoted to child development. It was not until the latter half of the 20th century that developmental scientists more seriously turned their attention to adult development and aging. As they did, several important challenges surfaced (see Elder, 1998; Settersten, 1999). The concepts and issues at stake with respect to children could not simply be extended to adults. And new and difficult questions were raised about continuity and change in adult lives over time, about social settings that structure movement through those years, about connections between lives, time, and place, and how to handle these complexities in theory and research. These remain the most important challenges for developmental scholarship in the 21st century. In addition, early models were developed under very different demographic conditions, and dramatic demographic change in the past century has set new and unknown parameters on human life.