The concept of bipolar spectrum disorder (BSD) has developed to include affective temperaments such as cyclothymia and hyperthymia. This has greatly helped clinicians to differentiate depressed patients, who would potentially benefit from mood stabilizing treatment, from those with unipolar depression. Cyclothymia, however, has significant similarities with personality disorders, especially with borderline personality disorder (BPD). All the diagnostic items for BPD are frequently found in patients with BSD as well, which presents diagnostic challenges. There are no clear guidelines on how to differentiate BSD from BPD. Featuring borderline pathology for clinical purposes, it may be useful to rely on psychodynamic approaches to identify primitive defense mechanisms of splitting and projective identification suggesting borderline personality organization. Based on new findings on common features between BSD and BPD, some authors have proposed a renewal of the classification system of mental disorders. The dichotomy of bipolar and unipolar depression has gestated a new concept of BSD. Currently, the BSD concept forced us to formulate the border of BSD and personality disorders.