The chapter introduces the concept of life span developmental psychology, which focuses on the study of individual development from conception to old age. It emphasizes that development is not completed at adulthood but extends throughout the entire life course, involving lifelong adaptive processes. The chapter outlines the historical context of life span psychology, highlighting the contributions of early scholars like Johann Nicolaus Tetens and Adolphe Quetelet, who recognized the lifelong nature of development and its interplay with sociocultural conditions. The chapter then discusses the organizational framework of life span theory, distinguishing between person-centered (holistic) and function-centered approaches. It explains that life span theory can be seen as a coordinated integration of various age-based developmental specializations into a cumulative framework. The chapter also explores the historical development of life span psychology, noting its different emphasis in different regions, such as Germany and North America. The chapter proceeds to outline five levels of analysis for psychological theories of life span development: 1. **Overall Architecture of Life Span Development**: This level considers the biological and cultural co-evolution and co-construction of life span development, emphasizing the incomplete and dynamic nature of the overall architecture. 2. **Life Span Changes in the Relative Allocation of Resources to Distinct Functions of Development**: This level focuses on the allocation of resources to growth, maintenance/recovery, and regulation of loss, highlighting the shift in resource allocation across the life span. 3. **Metatheoretical Propositions about Life Span Developmental Theory**: This level involves a set of overarching principles and propositions that guide life span developmental theory. 4. **An Example of a Systemic and Overall Theory of Life Span Development**: This level presents a concrete example of a systemic and overall theory, such as Selective Optimization with Compensation. 5. **More Molecular Phenomena and Functions**: This level delves into specific areas of psychological functioning, such as cognition, intelligence, personality, and the self. The chapter concludes by emphasizing the importance of understanding the biocultural co-evolution and co-construction in life span development, the shift in resource allocation, and the need for both biology and culture to cooperate in compensating for the不完备ness of the life span architecture.The chapter introduces the concept of life span developmental psychology, which focuses on the study of individual development from conception to old age. It emphasizes that development is not completed at adulthood but extends throughout the entire life course, involving lifelong adaptive processes. The chapter outlines the historical context of life span psychology, highlighting the contributions of early scholars like Johann Nicolaus Tetens and Adolphe Quetelet, who recognized the lifelong nature of development and its interplay with sociocultural conditions. The chapter then discusses the organizational framework of life span theory, distinguishing between person-centered (holistic) and function-centered approaches. It explains that life span theory can be seen as a coordinated integration of various age-based developmental specializations into a cumulative framework. The chapter also explores the historical development of life span psychology, noting its different emphasis in different regions, such as Germany and North America. The chapter proceeds to outline five levels of analysis for psychological theories of life span development: 1. **Overall Architecture of Life Span Development**: This level considers the biological and cultural co-evolution and co-construction of life span development, emphasizing the incomplete and dynamic nature of the overall architecture. 2. **Life Span Changes in the Relative Allocation of Resources to Distinct Functions of Development**: This level focuses on the allocation of resources to growth, maintenance/recovery, and regulation of loss, highlighting the shift in resource allocation across the life span. 3. **Metatheoretical Propositions about Life Span Developmental Theory**: This level involves a set of overarching principles and propositions that guide life span developmental theory. 4. **An Example of a Systemic and Overall Theory of Life Span Development**: This level presents a concrete example of a systemic and overall theory, such as Selective Optimization with Compensation. 5. **More Molecular Phenomena and Functions**: This level delves into specific areas of psychological functioning, such as cognition, intelligence, personality, and the self. The chapter concludes by emphasizing the importance of understanding the biocultural co-evolution and co-construction in life span development, the shift in resource allocation, and the need for both biology and culture to cooperate in compensating for the不完备ness of the life span architecture.