BPD is a complex and severe mental health disorder, with typical symptom onset during adolescence and presence of behavioural precursors in childhood, persisting into adulthood. BPD is characterised by a pervasive pattern of unstable interpersonal relationships, pronounced impulsive and self-damaging behaviour, unstable identity, and difficulties with emotional dysregulation, which substantially impact in an enduring way on quality of life and psychosocial functioning. The DSM-5 diagnosis of BPD requires the pervasive presence of a minimum of five out of nine symptoms. In the general population, BPD has a prevalence of around 6% and within populations of adult psychiatric inpatients, prevalence is around 20%. Most epidemiological surveys report no gender differences of BPD, yet studies of clinical populations typically report higher prevalence figures in women than in men. The different sex ratios in clinical and population samples may be explained by both assessment and sampling biases. Individuals with BPD commonly present with comorbid mental health disorders. In particular, around 90% of BPD cases are reported to have cooccurring mood disorders including depression and dysthymia, along with a high prevalence of substance use disorders in the range of 15% to 57%.