Personality disorders, formerly referred to as character disorders, are a class of personality types and behaviors that the American Psychiatric Association (APA) defines as "an enduring pattern of inner experience and behavior that deviates markedly from the expectations of the culture of the individual who exhibits it". Personality disorders are noted on Axis II of the Diagnostic and Statistical Manual of Mental Disorders, or DSM-IV-TR (fourth edition, text revision) of the American Psychiatric Association.

Personality disorders are also defined by the International Statistical Classification of Diseases and Related Health Problems (ICD-10), which is published by the World Health Organization. Personality disorders are categorized in ICD-10 Chapter V: Mental and behavioural disorders, specifically under Mental and behavioral disorders: 28F60-F69.29 Disorders of adult personality and behavior.

These behavioral patterns in personality disorders are typically associated with severe disturbances in the behavioral tendencies of an individual, usually involving several areas of the personality, and are nearly always associated with considerable personal and social disruption. Additionally, personality disorders are inflexible and pervasive across many situations, due in large part to the fact that such behavior is ego-syntonic (i.e. the patterns are consistent with the ego integrity of the individual) and are, therefore, perceived to be appropriate by that individual.

The onset of these patterns of behavior can typically be traced back to late adolescence and the beginning of adulthood and, in rarer instances, childhood. It is therefore unlikely that a diagnosis of personality disorder will be appropriate before the age of 16 or 17 years. General diagnostic guidelines applying to all personality disorders are presented below; supplementary descriptions are provided with each of the subtypes.

Diagnosis of personality disorders can be very subjective; however, inflexible and pervasive behavioral patterns often cause serious personal and social difficulties, as well as a general functional impairment. Rigid and ongoing patterns of feeling, thinking and behavior are said to be caused by underlying belief systems and these systems are referred to as fixed fantasies or "dysfunctional schemata" (Cognitive modules).