A mental disorder, also referred to as a mental illness or psychiatric disorder, is a behavioral or mental pattern that causes significant distress or impairment of personal functioning. A mental disorder is also characterized by a clinically significant disturbance in an individual’s cognition, emotional regulation, or behaviour.  It is usually associated with distress or impairment in important areas of functioning. There are many different types of mental disorders. Mental disorders may also be referred to as mental health conditions. Such features may be persistent, relapsing and remitting, or occur as single episodes. Many disorders have been described, with signs and symptoms that vary widely between specific disorders. Such disorders may be diagnosed by a mental health professional, usually a clinical psychologist or psychiatrist.

The causes of mental disorders are often unclear. Theories may incorporate findings from a range of fields. Mental disorders are usually defined by a combination of how a person behaves, feels, perceives, or thinks. This may be associated with particular regions or functions of the brain, often in a social context. A mental disorder is one aspect of mental health. Cultural and religious beliefs, as well as social norms, should be taken into account when making a diagnosis.

Services are based in psychiatric hospitals or in the community, and assessments are carried out by mental health professionals such as psychiatrists, psychologists, psychiatric nurses and clinical social workers, using various methods such as psychometric tests but often relying on observation and questioning. Treatments are provided by various mental health professionals. Psychotherapy and psychiatric medication are two major treatment options. Other treatments include lifestyle changes, social interventions, peer support, and self-help. In a minority of cases, there might be involuntary detention or treatment. Prevention programs have been shown to reduce depression.

In 2019, common mental disorders around the globe include depression, which affects about 264 million, bipolar disorder, which affects about 45 million, dementia, which affects about 50 million, and schizophrenia and other psychoses, which affects about 20 million people. Neurodevelopmental disorders include i.a. attention deficit hyperactivity disorder (ADHD), autism spectrum disorder (ASD), and intellectual disability of which onset occurs early in the developmental period. Stigma and discrimination can add to the suffering and disability associated with mental disorders, leading to various social movements attempting to increase understanding and challenge social exclusion.