In order to help people with a mental illness it is important to be able to understand and measure the severity of distressing and disabling experiences that can affect behavior. SCAN is a form of interview, found acceptable to patients, which provides a detailed and accurate picture of the "mental state". When analyzed by computer, SCAN provides diagnoses that are comparable wherever the system is used. The ninth 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. SCAN/PSE-10 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.