This chapter explains the concept of mentalizing, and discusses the intergenerational transmission of mentalizing impairments in the context of attachment disturbance. It also illustrates how this developmental psychopathology is exemplified in borderline personality disorder (BPD). The intergenerational transmission of insecure attachment is the converse of secure attachment in being mediated by poor parental mentalizing. Befitting the sheer complexity of symptomatology, one can recognize a diversity of developmental pathways to BPD. The chapter focuses on mentalizing impairments in attachment relationships as providing optimal leverage in treating BPD. Two key domains of deficits are linked to attachment-related mentalizing problems: affect regulation and effortful control of attention. Offering an attachment relationship, the psychotherapist puts the traumatized patient with BPD in a dilemma: stimulating attachment needs potentially evokes trauma-related affects and thereby inhibits the fundamental capacity on which psychotherapy depends, namely, mentalizing.