Book excerpt: This volume focuses on concepts central to the understanding of the key features of individuality which undergo significant transformations throughout the adolescent period: Personality, self, and ego. While rooted in distinct theoretical traditions, these three concepts, in combination, capture the core aspects of the formation of the individual's unique sense of self or identity, a psychosocial development fundamentally associated with adolescence. Consistent with the developmental-systems models of person-context relations at the forefront of current human development theory and research, the articles within this volume focus on the dynamic, reciprocal relations between youth and key socializing agents within their ecologies. Nevertheless, the articles represented in this volume illustrate that when attempting to understand the development of personality- and self-systems, scholars differ in the extent to which they place primary emphasis on the individual, on the context, or on the relationship between the two.