The Interpersonal psychoanalytic tradition has its roots in 20th-Century American pragmatism and humanism. Its primary innovations arose in reaction to the biologic, drive-based psychoanalytic orthodoxy of early and mid-20th-Century psychoanalysis. Interpersonal departures from Classical Freudian theory are characterized by Harry Stack Sullivan’s theory of development which emphasized the person as a social being whose essence lies in the developmental internalization of the myriad relationships of caregivers, family, significant others, society and culture. This theory grounded personality in the organization of a self-system, formed, motivated and protected by universal needs for self-esteem, rather than acceptable gratification of biological drives. Along with this new emphasis on the individual personality as primarily socially determined, Sullivan posited transference as a universal and inevitable interpersonal phenomenon of unique perceptions and distortions. This was a radical theoretical shift from the orthodox conception of transference that viewed the anonymous, well-analyzed clinician as an objective, all-knowing observer of the patient’s psyche. Sullivan’s therapeutic model of participant-observation introduced the now universally accepted notion of analysts' and patients' irreducible subjectivity. Early Interpersonalists contributed to reformulating gender theories by questioning what was taken as bedrock in psychoanalysis and by considering the ways culture and politics influence the clinical context as well as theory development. Frieda Fromm Reichmann, Clara Thompson, Erich Fromm and Rollo May, humanistic theorists and existential thinkers and clinicians, placed an emphasis on self-expression and the role of free will, personal agency and choice, focusing on the ways in which individuals avoid anxiety by opting for conformity over freedom. Sandor Ferenczi set the tone for Interpersonalists’ exploration of the plasticity and flexibility of psychoanalytic technique which emphasized a less hierarchical analytic relationship. These groundbreaking developments were significantly elaborated beginning in the 1960’s through present by such analysts as Benjamin Wolstein and Edgar Levenson. For many years the Interpersonal tradition was in conflict with other schools for its emphasis on the enactments of both participants in analytic engagement. Now, in the 21st Century, the Interpersonalists’ initiatives are so widely accepted and so intricately interwoven into contemporary psychoanalysis that the original sources of psychoanalysis’ radical shifts from drive theory and one-person psychology are often forgotten. Today the psychoanalytic culture of the Interpersonal tradition is not a single, bounded corpus of theory. It is, rather, an integration of and enthusiastic openness to the work of a broad spectrum of analysts and researchers whose work builds on their foundational ideas.