Using the same Diagnostic and Statistical Manual of Mental Disorders, fifth version (DSM-V) criteria as in adults, borderline personality disorder (BPD) in adolescents is defined as a 1-year pattern of immature personality development with disturbances in at least five of the following domains: efforts to avoid abandonment, unstable interpersonal relationships, identity disturbance, impulsivity, suicidal and self-mutilating behaviors, affective instability, chronic feelings of emptiness, inappropriate intense anger, and stress-related paranoid ideation. BPD can be reliably diagnosed in adolescents as young as 11 years. The available epidemiological studies suggest that the prevalence of BPD in the general population of adolescents is around 3%. The clinical prevalence of BPD ranges from 11% in adolescents consulting at an outpatient clinic to 78% in suicidal adolescents attending an emergency department. The diagnostic procedure is based on a clinical assessment with respect to developmental milestones and the interpersonal context. The key diagnostic criterion is the 1-year duration of symptoms. Standardized, clinician-rated instruments are available for guiding this assessment (eg, the Diagnostic Interview for Borderlines-Revised and the Childhood Interview for DSM-IV-TR BPD). The assessment should include an evaluation of the suicidal risk. Differential diagnosis is a particular challenge, given the high frequency of mixed presentations and comorbidities. With respect to clinical and epidemiological studies, externalizing disorders in childhood constitute a risk factor for developing BPD in early adolescence, whereas adolescent depressive disorders are predictive of BPD in adulthood. The treatment of adolescents with BPD requires commitment from the parents, a cohesive medical team, and a coherent treatment schedule. With regard to evidence-based medicine, psychopharmacological treatment is not recommended and, if ultimately required, should be limited to second-generation antipsychotics. Supportive psychotherapy is the most commonly available first-line treatment. Randomized controlled trials have provided evidence in favor of the use of specific, manualized psychotherapies (dialectic-behavioral therapy, cognitive analytic therapy, and mentalization-based therapy).