Schizophrenia is a psychotic illness that erodes the ability to initiate and organise self-directed mental activity and to recognise oneself as the source of such activity. It can produce a diverse array of disturbances within the domains of thought, perception, affect and volition. Typically, the illness follows a course in which acute episodes of hallucinations, delusions and florid disorganisation of thought are superimposed upon more persistent and subtle disorders of the initiation and organisation of thought and behaviour. These persistent disorders can profoundly disrupt occupational activities and social relationships. However, the severity of the persisting disorder varies greatly between cases.

The disorder we think of as schizophrenia is the archetypal exemplar of a spectrum of disorders that extends to schizoaffective disorder, delusional disorder and schizotypal disorder. While there have not been major changes in the specific diagnostic criteria for any of these specific disorders in the 11th edition of International Classification of Diseases, ICD-11, nor the 5th edition of the Diagnostic and Statistical Manual of the American Psychiatric Association, DSM-5, compared with the immediately preceding editions, a shift towards emphasis on regarding schizophrenia as one pole of a spectrum of psychotic disorders reflects an ongoing debate.