The introduction of the concept of mentalization in psychological science by Fonagy and colleagues has opened up new perspectives for the understanding of psychopathology, psychotherapy, and child development. The present study reviews the theory of mentalization, with a focus on its 4 dimensions (cognitive/affective, implicit/explicit, self/other, and external/internal), and some unclear points and unresolved issues are identified. Mentalization theory is then contrasted with the theory of primary intersubjectivity, which is often seen as an incompatible approach to the development of social understanding. It is argued that this theory, at least in 1 of its interpretations, is not only compatible with mentalization theory, but may also possibly contribute to the resolution of some problems in mentalization theory. More specifically, it is argued that mentalization originally develops in the context of primary intersubjectivity, and that primary intersubjectivity is a basic prerequisite for the development of mentalization; but also that there is a considerable overlap between the concepts of primary intersubjectivity and those of implicit and externally focused mentalization.