A mental disorder or mental illness is a psychological or behavioral pattern associated with distress or disability that occurs in an individual and is not a part of normal development or culture. The recognition and understanding of mental health conditions has changed over time and across cultures, and there are still variations in the definition, assessment, and classification of mental disorders, although standard guideline criteria are widely accepted.

Over a third of people in most countries report meeting criteria for the major categories at some point in their life. The causes are often explained in terms of a diathesis-stress model and biopsychosocial model. Services are based in psychiatric hospitals or in the community, and mental health professionals diagnose individuals by various methods, often relying on observation and questioning in interview.

Psychotherapy and psychiatric medication are two major treatment options as are social interventions, peer support and self-help. In some cases there may be involuntary detention and involuntary treatment where legislation allows. Stigma and discrimination add to the suffering associated with the disorders, and various social movements campaign for change.