Contemporary, cutting-edge scholarship in human developmental science is framed by developmental systems theories emphasizing that the basic process in development involves mutually influential relations between the individually distinct person and his or her diverse, multilevel context; stressing that relative plasticity in development derives from such individual ↔ context relations; and providing optimism about the possibility that applications of developmental science may promote positive development. These ideas underscore the substantive importance of human diversity, seen both as the potential for systematic intraindividual change across life (plasticity) and as interindividual differences in intraindividual change, and stand in contrast to views about diversity as either unimportant, error, or reflecting deficits in development. I discuss these differing conceptions of human diversity and of their distinct implications for the methodological and ethical conduct of science and of applications to programs and policies aimed at promoting positive development.