Two-polarities models of personality propose that personality development evolves through a dialectic synergistic interaction between two fundamental developmental psychological processes across the life span--the development of interpersonal relatedness on the one hand and of self-definition on the other. This article offers a broad review of extant research concerning these models, discusses their implications for psychology and psychiatry, and addresses future research perspectives deriving from these models. We first consider the implications of findings in this area for clinical research and practice. This is followed by a discussion of emerging research findings concerning the role of developmental, cross-cultural, evolutionary, and neurobiological factors influencing the development of these two fundamental personality dimensions. Taken together, this body of research suggests that theoretical formulations that focus on interpersonal relatedness and self-definition as central coordinates in personality development and psychopathology provide a comprehensive conceptual paradigm for future research in psychology and psychiatry exploring the interactions among neurobiological, psychological, and sociocultural factors in adaptive and disrupted personality development across the life span. (Contains 1 figure.)