Borderline personality disorder is characterised by a pervasive pattern of instability in affect regulation, impulse control, interpersonal relationships, and self-image. Clinical signs of the disorder include emotional dysregulation, impulsive aggression, repeated self-injury, and chronic suicidal tendencies, which make these patients frequent users of mental-health resources. Causal factors are only partly known, but genetic factors and adverse events during childhood, such as physical and sexual abuse, contribute to the development of the disorder. Dialectical behaviour therapy and psychodynamic partial hospital programmes are effective treatments for out-of-control patients, and drug therapy can reduce depression, anxiety, and impulsive aggression. More research is needed for the understanding and management of this disabling clinical condition. Current strategies are focusing on the neurobiological underpinnings of the disorder and the development and dissemination of better and more cost-effective treatments to clinicians.