This article addresses the question whether borderline personality disorder (BPD) can be understood as a variant of bipolar disorder. In the past, borderline pathology has been seen as a variant of psychosis, depression, or posttraumatic stress disorder, but there are important differences between all of these conditions and BPD. The proposal that BPD falls within the bipolar spectrum depends on the assumption that affective instability develops through the same mechanism in both diagnostic categories. There are major differences in phenomenology, family history, longitudinal course, and treatment response between BPD and bipolar disorder, and the findings of comorbidity studies are equivocal. Thus, existing evidence is insufficient to support the concept that BPD falls in the bipolar spectrum.