Borderline personality disorder (BPD) has onset in adolescence, but is typically first diagnosed in young adulthood. This paper provides a narrative review of the current evidence on diagnosis, comorbidity, phenomenology and treatment of BPD in adolescence. Instruments available for diagnosis are reviewed and their strengths and limitations discussed. Having confirmed the robustness of the diagnosis and the potential for its reliable clinical assessment, we then explore current understandings of the mechanisms of the disorder and focus on neurobiological underpinnings and research on psychological mechanisms. Findings are accumulating to suggest that adolescent BPD has an underpinning biology that is similar in some ways to adult BPD but differs in some critical features. Evidence for interventions focuses on psychological therapies. Several encouraging research studies suggest that early effective treatment is possible. Treatment development has just begun, and while adolescent-specific interventions are still in the process of evolution, most existing therapies represent adaptations of adult models to this developmental phase. There is also a significant opportunity for prevention, albeit there are few data to date to support such initiatives. This review emphasizes that there can be no justification for failing to make an early diagnosis of this enduring and pervasive problem.