Internalization occupies a pivotal position across the depth-psychological corpus, functioning simultaneously as a developmental mechanism, a therapeutic goal, and — for certain theorists — an epistemological problem. The classical psychoanalytic tradition, represented here by Hartmann, Loewenstein, Loewald, Schafer, and Meissner, defines internalization variously as the transformation of external regulations into internal ones, the conversion of external relations into internal structure, and the ‘taking in’ of absent others. Schore grounds this lineage neurobiologically, arguing that the caregiver’s regulatory function is permanently inscribed in the child’s right hemisphere and that psychotherapy seeks to recreate this ontogenetic process through a series of attachments and separations. Klein anchors the concept even earlier, treating the internalization of the good object as the very basis of postnatal development. Object-relations clinicians such as Flores extend the discussion into treatment of addiction, showing how transmuting internalization builds psychic structure where early self-object failure left deficits. Vygotsky’s zone of proximal development enters via Schore, adding a sociocognitive dimension. Against these structuralist positions, Schwartz (IFS) mounts a pointed critique: the ‘myth of environmental dependency’ overstates the role of internalization and underestimates the innate Self. Hillman, characteristically oblique, warns that ‘internalizing can become just as literal as acting out,’ insisting that psychic movement inward must remain imaginal rather than mechanical. Cairns traces cross-cultural debates about whether shame cultures achieve genuine internalization of norms or merely contingent compliance. The term thus marks a fault line between constructivist, relational, neurobiological, and archetypal models of psychic formation.