The terms mentalization and mentalizing have recently emerged in the psychoanalytic and psychodynamic literature, especially in relation to the treatment of people suffering from borderline personality disorder. This paper discusses the meaning, phenomenology, and clinical applications of the concept. Mentalizing can be related to similar ideas developed in relation to autism under the heading of ‘theory of mind’; Bion's alpha function in relation to deficits in thinking in people with psychotic processes; and a body of francophone psychoanalytic literature related to people suffering from somatization disorders who have difficulty in finding a language to describe emotions. In focusing on failure of mentalizing in people with borderline personality disorder, Fonagy and his co-workers have put forward a developmental theory in which the ability to monitor one's own and others' thought processes is compromised. Clinical examples are described and the advent of this new concept into the psychoanalytic vernacular is welcomed.