The Unified Medical Language System (UMLS) is a comprehensive repository of biomedical vocabularies developed by the US National Library of Medicine. It integrates over 2 million names for approximately 900,000 concepts from more than 60 families of biomedical vocabularies, along with 12 million relations among these concepts. The UMLS Metathesaurus includes major resources such as the NCBI taxonomy, Gene Ontology, Medical Subject Headings (MeSH), OMIM, and the Digital Anatomist Symbolic Knowledge Base. UMLS concepts are interconnected and can be linked to external resources like GenBank. The UMLS provides tools for customizing the Metathesaurus, generating lexical variants, and extracting concepts from text. These tools include MetamorphoSys, lvg, and MetaMap. The UMLS knowledge sources are updated quarterly and are available for research purposes within institutions, with a license agreement required for use. The UMLS is designed to facilitate interoperability and integration of biomedical resources, making it a valuable resource for bioinformaticians and researchers.The Unified Medical Language System (UMLS) is a comprehensive repository of biomedical vocabularies developed by the US National Library of Medicine. It integrates over 2 million names for approximately 900,000 concepts from more than 60 families of biomedical vocabularies, along with 12 million relations among these concepts. The UMLS Metathesaurus includes major resources such as the NCBI taxonomy, Gene Ontology, Medical Subject Headings (MeSH), OMIM, and the Digital Anatomist Symbolic Knowledge Base. UMLS concepts are interconnected and can be linked to external resources like GenBank. The UMLS provides tools for customizing the Metathesaurus, generating lexical variants, and extracting concepts from text. These tools include MetamorphoSys, lvg, and MetaMap. The UMLS knowledge sources are updated quarterly and are available for research purposes within institutions, with a license agreement required for use. The UMLS is designed to facilitate interoperability and integration of biomedical resources, making it a valuable resource for bioinformaticians and researchers.