This paper reviews reliability and validity studies of the WHO - Composite International Diagnostic Interview (CIDI). The CIDI is a comprehensive and fully standardized diagnostic interview designed for assessing mental disorders according to the definitions of the Diagnostic Criteria for Research of ICD-10 and DSM-III-R. The instrument contains 276 symptom questions many of which are coupled with probe questions to evaluate symptom severity, as well as questions for assessing help-seeking behavior, psychosocial impairments, and other episode-related questions. Although primarily intended for use in epidemiological studies of mental disorders, it is also being used extensively for clinical and other research purposes. The review documents the wide spread use of the instrument and discusses several test-retest and interrater reliability studies of the CIDI. Both types of studies have confirmed good to excellent Kappa coefficients for most diagnostic sections. In international multicenter studies as well as several smaller center studies the CIDI was judged to be acceptable for most subjects and was found to be appropriate for use in different kinds of settings and countries. There is however still a need for reliability studies in general population samples, the area the CIDI was primary intended for. Only a few selected aspects of validity have been examined so far, mostly in smaller selected clinical samples. The need for further procedural validity studies of the CIDI with clinical instruments such as the SCAN as well as cognitive validation studies is emphasized. The latter should focus on specific aspects, such as the use of standardized questions in the elderly, cognitive probes to improve recall of episodes and their timing, as well as the role of order effects in the presentation of diagnostic sections.