RÉSUMÉ
The Anatomical Therapeutic Chemical (ATC) classification system and the Defined Daily Dose (DDD) as a measuring unit was developed along with an increased concern about drug utilization studies in Europe in the 1960s. Its use was recommended by the WHO Regional Office of Europe on 1981, then by the WHO Headquarters in Geneva in 1996. It is maintained by the WHO Collaborating Centre on Drug Statistics Methodology in Olso (http://www.whocc.no/) and widely used in Europe. However, it is rarely used in other parts of the world. This paper aims to inform the Japanese public about the ATC/ DDD system towards a more rational use of drugs in Japan. It attempts to answer the five Ws on ATC/ DDD, i.e., “What is ATC/DDD?”; “Why is ATC/DDD used?”; “When was ATC/DDD established?”; “Who decide ATC/DDD and how?”; and “Where is ATC/DDD used?”