This chapter discusses the gains in function made during adolescence, a time of major upheaval in behavioral and social domains. It reviews the structural brain changes that occur during this time and explores neuropsychological development, with a special focus on executive functions. The chapter discusses links between cognitive and cerebral development, with an emphasis on the impact of developmental lesions. Finally, it presents a hypothesis explaining the neuropsychological deficits in schizophrenia as an interaction between the timing of illness onset and the timing of normal cognitive development. Specifically, the chapter suggests that cognitive functions that mature around the time when the illness first presents, such as working memory, are more impaired than those functions that mature earlier. The study of cognitive development through adolescence is of great importance to the understanding of the neurobiology of disorders that first present at this time.