Borderline personality disorder (BPD) is the most common and the most serious of the personality disorders (PDs) in clinical practice. This chapter overviews sex differences in BPD, covering clinical presentation, longitudinal course, aetiological factors and neurobiological underpinnings. Cross-sectional baseline data from the Collaborative Longitudinal Study of Personality Disorders (CLPS) comparing 175 women with 65 men with BPD found men with BPD to be more likely to present with substance use disorders, along with schizotypal, narcissistic and antisocial personality disorder (ASPD). The aetiological underpinnings of sex differences in PDs in general and BPD in particular are complex, with multi-level interactions of genetic and environmental parameters acting at certain vulnerable stages of neural, emotional and social development. Neuropsychological investigation suggests a dysfunctional prefrontal circuit in impulsive aggression. Three positron emission tomography (PET) studies have found prefrontal cortical hypo metabolism in BPD compared to healthy controls.