Background: The relationship between bipolar disorder (BD) and borderline personality disorder (BPD) has been controversial and widely debated. Specifically, the comorbidity between both disorders has yielded a plethora of research, but there are no comprehensive reviews on this issue. Objective: To determine the empirical evidence regarding the comorbidity between BD and BPD based on prevalence data, explanatory theories for their co-occurrence, and clinical impact of one disorder in the other. Method: A comprehensive search of databases (PubMed and PsycINFO) was performed. Published manuscripts between January 1985 and August 2015 were identified. Overall, 70 studies fulfilled inclusion criteria. Results: Over a fifth of subjects showed comorbidity between BPD and BD. Empirical evidence from common underlying factors was inconclusive, but BPD appears to be a risk factor for BD. Data also indicated that the negative impact of BPD in BD (e.g., suicidality, worse mood course) was greater than vice verse. Conclusions: Given the high prevalence of comorbidity between BD and BPD and the negative effects of BPD in subjects with BD, further studies are needed to clarify the factor associated with the comorbidity between these two disorders. This information is important to develop appropriate treatments for subjects with both disorders, improve their clinical course, and prevent the increased risk of suicidality commonly found in these subjects. (C) 2016 Elsevier B.V. All rights reserved.