This article discusses some of Irene Fast's notable contributions to fundamental psychoanalytic issues. In nearly a half century of writing, at a time of significant transformation in psychoanalytic thinking and clinical work, Fast's ideas about differentiation processes in development, the formation of gender identity, and how the mind is organized and structured have been highly influential. Fast's precise, clear thinking, and her ability to get inside of the developing mind have been at the cutting edge of the interface between intrapsychic and Interpersonal/Relational ideas, while anchored in a serious appreciation of Freud's work. Fast was one of my first supervisors, and later a mentor and personal friend. Many of Fast's ideas have influenced mine, and this article describes how an application of Fast's views can significantly enhance psychoanalytic understanding of transference and of the “reversibility” of affective experience between patient and therapist in transference-countertransference dynamics.