The principles underlying the creation and long-term development of the Present State Examination (PSE), now incorporated, with its glossary of definitions of symptoms and other components, in a shell known as the Schedules for Clinical Assessment in Neuropsychiatry (SCAN), are derived from standards of clinical assessment gradually developed from the late 18th century onwards. A form of ‘mental state examination’ is now an established part of the standard curriculum in most medical schools. To conduct such an examination requires the clinician to meet a demanding list of requirements. The first essential is a sound grasp of clinical psychopathology. This confers an ability to recognise, and distinguish within and between, an extensive range of symptoms, such as delusions, hallucinations, obsessional ruminations, irrational fears of harmless stimuli and feelings of excessive guilt. A technique of ‘cross-examination’, based on the glossary of differential definitions of symptoms, must be learned in the first place by observing experienced practitioners interviewing their patients. It is only after completing a comprehensive clinical symptom base that diagnostic procedures should, if needed, be brought into play with the help of algorithms laid down in international standards, as well as those of any other system in local use. These specifics differentiate SCAN from methods such as fully structured questionnaires or self-report forms, which have other purposes.