In this chapter, we argue that development is a process of participation in sociocultural activities. We regard individual development as inseparable from interpersonal and community processes; individuals' changmg roles are mutually defined with those of other people and with dynamic cultural processes. We make use of "activity" or "event" as the unit of analysis, with active and dynamic contributions from individuals, their social partners, and historical traditions and materials and their transformations (see Dewey and Bentley, 1949; Leontiev, 1981).Studymg human events or activities contrasts with the more traditional approach of examining the individual in isolation or in interaction with a separate environment. In our approach, individuals' efforts and sociocultural institutions and practices are constituted by and constitute each other and thus cannot be defined independently of each other or studied in isolation. We may focus on the contribution of one or another individual or a cultural tradition, but always in relation to the whole activity rather than extracted from it. When individuals participate in shared endeavors, not only doesWe are grateful to the Spencer Foundation for its support of the research reported here; to the Ministerio de Educaci6n