This paper describes a mentalization-based model of the development of borderline personality disorder (BPD). The model takes into account constitutional vulnerability and is rooted in attachment theory and its elaboration by contemporary developmental psychologists. The model suggests that disruption of the attachment relationship early in development in combination with later traumatic experiences in an attachment context interacts with neurobiological development. The combination leads to hyper-responsiveness of the attachment system which makes mentalizing, the capacity to make sense of ourselves and others in terms of mental states, unstable during emotional arousal. The emergence of earlier modes of psychological function at these times accounts for the symptoms of BPD. The model has clinical implications and suggests that the aim of treatment is not only to encourage development of mentalizing but also to facilitate its maintenance when the attachment system is stimulated.