This thesis aims to review the concepts of mentalization and the theory of mind, and their application in the understanding of psychological functioning and treatment of various psychiatric disorders such as borderline personality disorder and eating disorders. While the theory of mind is more of a philosophic and scientific concept, its conjunction with psychoanalytic ideas, such as attachment theory, thus constituting mentalization, concerns the interpersonally complex understandings of oneself and others, and the reflective abilities that enable an individual to face the social world and develop a stable sense of self. Mentalization is organized around three dichotomous dimensions: self-others, implicit-explicit and cognitive-affective. Partially overlapping concepts such as symbol formation, mindfulness, psychological mindedness, empathy and affect consciousness have been developed throughout psychoanalytic history. Mentalization is assessed, though not with full validity, by the measure of reflective function, a scale that is to be implemented in the Adult Attachment Interview – a semistructured clinical interview. Although the concept of mentalization is a useful tool to implement a valid treatment approach, it may be too broad and multidimensional to be operationalized as a marker for specific disorders such as borderline personality disorder or eating disorders. Additional research is required to clarify its benefits, disadvantages and differences with overlapping concepts. By reviewing recent and more historically available literature, an analysis of this complex subject is given in this thesis.