Borderline personality disorder (BPD) has always been considered an elusive and puzzling phenomenon. Concept and theory are indeed anything but straightforward. The ‘personality disorder’ construct appears to suggest a pathological condition located solely in the client. At the same time, the borderline dynamic is most famously associated with diﬃcult or unstable relationships and evokes images of harassed and tormented therapists. But borderline relationships are challenging for clients and therapists alike. Both may feel attacked, invaded, helpless, misunderstood or unappreciated by the other. Borderline patterns of organisation are evidently active across the continuum of intrapsychic and interpersonal ﬁelds. The word borderline – ‘a line that indicates a boundary’ – incidentally names what is most lacking in the borderline dynamic.