The Unified Medical Language System (UMLS) is a repository of biomedical vocabularies developed by the US National Library of Medicine. It integrates over 2 million names for 900,000 concepts from more than 60 biomedical vocabularies, along with 12 million relations among these concepts. The UMLS Metathesaurus includes major biomedical vocabularies such as NCBI taxonomy, Gene Ontology, MeSH, OMIM, and the Digital Anatomist Symbolic Knowledge Base. UMLS concepts are interconnected and may be linked to external resources like GenBank. In addition to data, the UMLS provides tools for customizing the Metathesaurus (MetamorphoSys), generating lexical variants (Ivg), and extracting UMLS concepts from text (MetaMap). The UMLS knowledge sources are updated quarterly and are available for research purposes at no cost, but users must sign a license agreement. The UMLS is accessible via CD-ROM and FTP, and its relational files can be loaded into a local database for SQL queries. An object-oriented model and XML-based queries are also available. The UMLS also includes tools such as MetamorphoSys, lvg, and MetaMap. These tools help users customize, generate lexical variants, and extract concepts from text. The UMLS is a comprehensive resource for biomedical terminology, integrating various subdomains such as anatomy, genetics, and clinical information systems. It provides a structured framework for organizing and relating biomedical concepts, enabling efficient information retrieval and integration. The UMLS is widely used in bioinformatics and biomedical research for its extensive coverage and integration of diverse terminologies.The Unified Medical Language System (UMLS) is a repository of biomedical vocabularies developed by the US National Library of Medicine. It integrates over 2 million names for 900,000 concepts from more than 60 biomedical vocabularies, along with 12 million relations among these concepts. The UMLS Metathesaurus includes major biomedical vocabularies such as NCBI taxonomy, Gene Ontology, MeSH, OMIM, and the Digital Anatomist Symbolic Knowledge Base. UMLS concepts are interconnected and may be linked to external resources like GenBank. In addition to data, the UMLS provides tools for customizing the Metathesaurus (MetamorphoSys), generating lexical variants (Ivg), and extracting UMLS concepts from text (MetaMap). The UMLS knowledge sources are updated quarterly and are available for research purposes at no cost, but users must sign a license agreement. The UMLS is accessible via CD-ROM and FTP, and its relational files can be loaded into a local database for SQL queries. An object-oriented model and XML-based queries are also available. The UMLS also includes tools such as MetamorphoSys, lvg, and MetaMap. These tools help users customize, generate lexical variants, and extract concepts from text. The UMLS is a comprehensive resource for biomedical terminology, integrating various subdomains such as anatomy, genetics, and clinical information systems. It provides a structured framework for organizing and relating biomedical concepts, enabling efficient information retrieval and integration. The UMLS is widely used in bioinformatics and biomedical research for its extensive coverage and integration of diverse terminologies.