This chapter, titled "Lifespan Psychology: Theory and Application to Intellectual Functioning," is part of a collection on disciplinary approaches to aging, edited by Donna Lind Infeld. The chapter focuses on the theoretical and empirical aspects of lifespan psychology, particularly in the context of cognitive development. It integrates evolutionary and ontogenetic perspectives to understand cultural and human development across various levels of analysis. The authors propose a meta-theory of development based on the interplay between selection, optimization, and compensation, which are essential processes in lifespan development. They argue that successful development involves the maximization of gains and the minimization of losses, with these processes being person-specific and contextually bound. The chapter also discusses the two-component model of lifespan intellectual development, distinguishing between the mechanics and pragmatics of cognition. The mechanics of cognition are associated with neurophysiological processes, while the pragmatics are linked to acquired knowledge and cultural resources. Empirical evidence supports the two-component model, showing that different cognitive abilities show differential age-related changes, with some abilities showing monotonic decline and others showing more stable or even age-related improvements. The chapter concludes by highlighting the importance of understanding the dynamics between gains and losses in lifespan development and the role of compensatory strategies in maintaining and enhancing cognitive function across the lifespan.This chapter, titled "Lifespan Psychology: Theory and Application to Intellectual Functioning," is part of a collection on disciplinary approaches to aging, edited by Donna Lind Infeld. The chapter focuses on the theoretical and empirical aspects of lifespan psychology, particularly in the context of cognitive development. It integrates evolutionary and ontogenetic perspectives to understand cultural and human development across various levels of analysis. The authors propose a meta-theory of development based on the interplay between selection, optimization, and compensation, which are essential processes in lifespan development. They argue that successful development involves the maximization of gains and the minimization of losses, with these processes being person-specific and contextually bound. The chapter also discusses the two-component model of lifespan intellectual development, distinguishing between the mechanics and pragmatics of cognition. The mechanics of cognition are associated with neurophysiological processes, while the pragmatics are linked to acquired knowledge and cultural resources. Empirical evidence supports the two-component model, showing that different cognitive abilities show differential age-related changes, with some abilities showing monotonic decline and others showing more stable or even age-related improvements. The chapter concludes by highlighting the importance of understanding the dynamics between gains and losses in lifespan development and the role of compensatory strategies in maintaining and enhancing cognitive function across the lifespan.