In order to help people with a mental illness it is important to be able to understand and measure the severity of the experiences that they find distressing and disabling and which can affect their behaviour. SCAN is a form of interview, found acceptable to patients, which provides a detailed and accurate picture of ‘mental state’. When analysed by computer it provides a diagnosis that is comparable wherever the system is used. The 9th edition of the Present State Examination (PSE-9) was published in 1974 and served to link the two main approaches to the description and classification of psychological problems. PSE-10/SCAN builds on the experience of extensive tests using PSE-9. It retains the main features of PSE-9 and links together the latest two international classification systems - ICD-10 and DSM-IV. This reference manual is a companion to the SCAN interview schedule and software. It describes the rationale and development of the system and provides a valuable introduction to its uses.