The assessment and quantification of tics can pose considerable difficulties because of a number of factors, including the spontaneous fluctuations of tic severity and the tendency of patients to suppress their tics, especially when in the consultation room with the clinician. It is therefore advisable to supplement direct observation and clinical interviewing (including historical and external information whenever available) with the use of psychometric instruments validated in patients with Tourette syndrome. Information can be obtained from patients or from informants such as parents and teachers, through questionnaire constructs. For Tourette syndrome, most available instruments are parent- and/or teacher-rated questionnaires in childhood and self-report scales in adolescence and adulthood (Table 2.1).