The relational unconscious — addressed in the corpus variously as unconscious-to-unconscious interaction, participation mystique, and implicit relational knowing — designates the dimension of psychic life that operates between subjects rather than solely within a single subject. The depth-psychology corpus approaches this phenomenon from at least three distinct orientations. In the Jungian lineage, Jung’s own recognition (elaborated by Stein, Jacoby, Wiener, and Sedgwick) that transference and countertransference are not merely dyadic communications between conscious parties but constitute a subterranean relationship between two unconscious systems — his so-called a′-to-b′ couple — establishes the foundational thesis: something psychically real transpires at a level that may never be fully analyzed. Neumann’s theorization of participation mystique extends this into developmental and anthropological registers, locating the relational unconscious at the origins of consciousness itself, prior to the differentiation of subject and object. A third current, represented by Ogden (Pat) and the sensorimotor-psychotherapy tradition, approaches the same territory through the concept of implicit relational knowing — procedural, somatic, and pre-reflective patterns that shape dyadic encounter beneath linguistic exchange. Tensions persist between those who privilege the archetypal-symbolic dimension of this unconscious communion (Jung, Alcaro) and those who theorize it in terms of attachment, embodiment, and intersubjective neuroscience. That the phenomenon resists full consciousness — by definition — is the shared conviction across these otherwise divergent positions.