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.

Currently, mental disorders are conceptualized as disorders of brain circuits likely caused by developmental processes shaped by a complex interplay of genetics and experience. In other words, the genetics of mental illness may really be the genetics of brain development, with different outcomes possible, depending on the biological and environmental context.

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. Diagnoses are made by psychiatrists or clinical psychologists using various methods, often relying on observation and questioning in interviews. Treatments are provided by various mental health professionals.

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 have led to various social movements campaign for change. Most recently, the field of Global Mental Health has emerged, which has been defined as 'the area of study, research and practice that places a priority on improving mental health and achieving equity in mental health for all people worldwide'.