The diagnostic criteria for mental health disorders are defined in internationally accepted diagnostic manuals. Research settings most commonly use the Diagnostic and Statistical Manual of Mental Disorders (DSM), published by the American Psychiatric Association. Although the fifth version of this manual (DSM-5) was in use during the development of these guidelines, the large majority of studies included in the literature reviews relied on DSM-IV-TR criteria for mental disorders. As such, we refer to both DSM-IV-TR and DSM-5 criteria where appropriate. The other major classification system is the International Classification of Diseases (ICD), with the current version being ICD-10. ICD-11 is currently under development. Although widely used in clinical practice in many parts of the world, ICD is rarely used in the type of research studies included for review. Nevertheless, ICD is referred to in these guidelines where appropriate.