The aim of this article is to describe the normal and abnormal development of mentallisation and development of borderline personality disorder as a mentallisation dysfunction, in the context of development of attachment relation. Borderline personality disorder is a distinct clinical syndrome with important implications for public health; patients show reduced capacities to mentalise, which inevitably leads to problems in interpersonal relationships, identity disturbance, impulsivity, emotional regulation and suicidal threats. In this article author present a review of a construct of mentallisation as it is developed by P. Fonagy and his collaborators. Mentalising is the process of making sense of mental states in oneself and other persons in terms of subjective states and mental processes. The concept of mentallisation is rooted in attachment theory. The author reviews the biobehavioural switch-model of the relationship between attachment, arousal or stress, mentallisation, and development of borderline personality disorder.