This groundbreaking book provides students and researchers with a unique overview of the longitudinal study of the development of young people from the ages of 12 to 25. It offers a comprehensive introduction into the multiple theories on the development of the self, personal relationships and psychopathology in adolescence, alongside a non-statistical overview of the many longitudinal models used to study development. The book includes key topics such as the development of the self, adolescent identity and personality; the development of parent-adolescent relationships; friendships and the understanding of others; and the development of psychosocial problems such as anxiety, depression, delinquency, aggression, and substance use. Meeus highlights multiple findings showing how these processes are integrated and identifies eight fundamental patterns of adolescent development to help determine why most adolescents develop into mature and organized individuals towards the end of this life stage, whilst a substantial minority show an inability to mature. It is essential reading for graduate students and researchers in adolescent development and anyone seeking to use longitudinal research methodology in the social and behavioral sciences.