Developmental psychology is a vast and fragmented discipline that is in dire need of a theoretical framework for thinking systematically about development. This book offers an integrative theoretical framework for thinking systematically about what happens during life span development and how life span development happens. Systems theory provides an overarching approach for conceptualizing the functioning and development of the whole person in terms of multiple and interrelated individual, social, and cultural processes. It is argued that much of what develops during development is cultural action in relation to others and that such action involves interrelations among varied domains of functioning (e.g., perception, cognition, emotion, language, social interaction, self/identity). Based on organismic-developmental theory, this conceptualization posits that development occurs in relation to cultural expectations and that action undergoes differentiation and integration during development. This conceptualization of what happens during development explicitly distinguishes between change and development and emphasizes dynamic developmental processes rather than static milestones. It is then creatively synthesized with sociocultural theory to explain how development happens through individual, social, and cultural processes. Varied examples from diverse cultures are used to explain central theoretical points and to illustrate how to use the theoretical framework to think systematically about human development. By embracing complexity and variability, the books conceptualization of action and development provides ways to address some of developmental psychologys most vexing issues and opens up exciting new directions for investigating the dynamics of human action and development.