The article presents a unified theory of development that integrates nature and nurture through a dialectical perspective, emphasizing the interconnectedness of individual and context. It discusses the historical evolution of developmental science, highlighting the alternating emphasis on nature and nurture as primary explanations for individual differences. The author argues that a more sophisticated understanding of human development requires recognizing the dynamic, reciprocal interactions between individuals and their environments. The article outlines four essential models for understanding human growth: personal change, context, regulation, and representation. The personal change model addresses the progression of competencies from infancy to adulthood, while the contextual model emphasizes the multiple sources of experience that influence development. The regulation model highlights the dynamic systems perspective, where individuals and their environments continuously interact and influence each other. The representational model focuses on how individuals encode and interpret their experiences, forming cognitive, social, and cultural structures. The author also explores nonlinear processes, such as the developmental helix, which illustrates the cyclical and evolving nature of development. The concept of differentiation and hierarchic integration is discussed, showing how developmental processes move from simplicity to complexity and back. The article emphasizes that neither nature nor nurture alone can fully explain development, as they are interdependent and mutually constitutive. The transactional model is introduced to describe the bidirectional interactions between individuals and their environments, highlighting the importance of mutual influence in shaping development. The article concludes by advocating for a unified theory of development that integrates these models and perspectives, providing a comprehensive framework for understanding human growth and adaptation.The article presents a unified theory of development that integrates nature and nurture through a dialectical perspective, emphasizing the interconnectedness of individual and context. It discusses the historical evolution of developmental science, highlighting the alternating emphasis on nature and nurture as primary explanations for individual differences. The author argues that a more sophisticated understanding of human development requires recognizing the dynamic, reciprocal interactions between individuals and their environments. The article outlines four essential models for understanding human growth: personal change, context, regulation, and representation. The personal change model addresses the progression of competencies from infancy to adulthood, while the contextual model emphasizes the multiple sources of experience that influence development. The regulation model highlights the dynamic systems perspective, where individuals and their environments continuously interact and influence each other. The representational model focuses on how individuals encode and interpret their experiences, forming cognitive, social, and cultural structures. The author also explores nonlinear processes, such as the developmental helix, which illustrates the cyclical and evolving nature of development. The concept of differentiation and hierarchic integration is discussed, showing how developmental processes move from simplicity to complexity and back. The article emphasizes that neither nature nor nurture alone can fully explain development, as they are interdependent and mutually constitutive. The transactional model is introduced to describe the bidirectional interactions between individuals and their environments, highlighting the importance of mutual influence in shaping development. The article concludes by advocating for a unified theory of development that integrates these models and perspectives, providing a comprehensive framework for understanding human growth and adaptation.