Personality disorders (PDs) are characterized by significant impairments to the self and interpersonal functioning, alongside pathological personality traits. One in ten people in the general population and one in two in clinical settings meet the diagnostic criteria for a PD. Comorbidity with other mental disorders is common, and the presence of a PD is the strongest explanation for recurrence, relapse, and poor prognosis in mental disorders. However, many individuals with a PD remain undetected in clinical practice and, as a result, are given ineffective or even harmful treatment. The ICD-11 classification of PDs represents a paradigm shift, replacing all PD subcategories with a single general description and differentiating individual PD expressions according to severity and personality trait domains. This classification is aligned with the DSM-5 Alternative Model for Personality Disorders, though the latter is not intended for general use. In addition to discussing the aforementioned issues, this chapter reviews the clinical descriptions of and empirical evidence for PDs (discussing them individually, in line with the DSM-5 classification) and presents meta-analytic data on the effectiveness of evidence-based psychotherapy for PDs.