Sidney Blatt's seminal contributions in the domain of personality development, psychopathology, and health rank among the best researched and most empirically supported theories in psychoanalysis. Blatt is known primarily for his two-polarities model of personality development, which he viewed as evolving through a dialectical, synergistic interaction between two fundamental processes across the lifespan: the development of interpersonal relatedness on the one hand, and of self-definition on the other. In this model, psychopathology is viewed as an attempt to find a balance, however distorted, between relatedness and self-definition. Neurobiological research has confirmed the intrinsic dialectical relationship between these two processes in the development of the neural circuits subserving these capacities, a finding with important implications for physical health. Research relevant to these ideas is reviewed, and the influence that Blatt's approach has had in reintroducing psychodynamic factors into contemporary psychology and psychiatry, as reflected in DSM-5, is discussed.