Borderline personality disorder (BPD) is a mental disorder with a high burden on patients, family members, and health-care systems. The condition was previously regarded as untreatable, but progress in understanding and management has resulted in earlier diagnosis and better treatment outcomes. A coherent syndrome of BPD typically onsets during adolescence (after age 12 years). BPD is often preceded by or co-develops with symptoms of internalising disorders (depression and anxiety), externalising disorders (conduct problems, hyperactivity, and substance use), or both. BPD is associated with various poor outcomes, including low occupational and educational attainment, lack of long-term relationships, increased partner conflict, sexual risk-taking, low levels of social support, low life satisfaction, and increased service use. Psychotherapy is the main treatment for BPD; drug treatment is only indicated for comorbid conditions that require medication, or during a crisis if psychosocial interventions are insufficient. Awareness of BPD by non-specialists, as well as specialists, is key to appropriate early intervention.