In recent years, advances in the areas of both bipolar and borderline personality disorders have generated considerable interest in the clinical interface between these two conditions. Developments in the study of the neurobiology of borderline personality disorder suggest that many patients with this diagnosis have etiological features in common with those diagnosed with bipolar disorders. This claim is supported by new insights into the phenomenology of both disorders and by evidence that mood stabilizers are efficacious in the pharmacological management of borderline patients. This area of research is an important one because of the considerable morbidity and public health costs associated with borderline personality disorder. Since borderline patients can be so challenging to care for, it may be that a reframing of the disorder as belonging to the broad clinical spectrum of bipolar disorders holds benefits for patients and clinicians alike.