This article provides an overview of definitions and assessment instruments of disability, an important topic in the diagnosis and treatment of chronic illnesses, and in expert appraisals in social and forensic medicine. Health problems are manifested not only in symptoms, but also regularly in impairment or disability in everyday life, which is especially true for mental disorders. According to the International Classification of Functioning, Disability and Health (ICF) of the World Health Organization, disability can be understood as chronic suffering from symptoms of illness, or limitations of executing capacities, or inability to participate in selected areas of life. Operationally, disability can be defined as “capacity limitations which hinder the ability to execute needed activities and thereby participation in a given environment”. This capacity–context–interaction model shows that there is no general disability but only context‐related disability, which has manifold consequences for diagnosis and care. Copyright © 2017 John Wiley & Sons, Ltd.