Various controlled vocabularies such as thesaurus and classification used for effective information retrieval contain different terms in expressing the same concept or meaning. National Library of Medicine has developed the Unified Medical Language System(UMLS) to solve the problems of information retrieval and integration resulted from the difference of concepts between different sources. The UMLS development was initiated in 1982 as a long-term project, and the 2001 edition of the UMLS consists of three parts : Metathesaurus, Semantic Network, and SPECIALIST Lexicon. This paper reviews background and structure of the UMLS including applications in PubMed, NLM Gateway.