The classification of mental disorders is also known as psychiatric nosology or psychiatric taxonomy. It represents a key aspect of psychiatry and other mental health professions and is an important issue for people who may be diagnosed. There are currently two widely established systems for classifying mental disorders:

• Chapter V of the tenth International Classification of Diseases (ICD-10) produced by the World Health Organization (WHO);
• The Diagnostic and Statistical Manual of Mental Disorders (DSM-5) produced by the American Psychiatric Association (APA).

Both list categories of disorders thought to be distinct types, and have deliberately converged their codes in recent revisions so that the manuals are often broadly comparable, although significant differences remain. Other classification schemes may be in use more locally, for example the Chinese Classification of Mental Disorders. Other manuals have some limited use by those of alternative theoretical persuasions, such as the Psychodynamic Diagnostic Manual.

The widely used DSM and ICD classifications employ operational definitions.