The Human Metabolome Database (HMDB) is a comprehensive, web-accessible resource that provides detailed information on human metabolites, their associated enzymes, transporters, abundance, and disease-related properties. Since its first release in 2007, HMDB has been used in nearly 100 published studies in metabolomics, clinical biochemistry, and systems biology. The latest version, HMDB 2.0, has significantly expanded and enhanced the database. The number of fully annotated metabolite entries has increased from 2180 to over 6800, a 300% increase. The number of metabolites with bio-fluid or tissue concentration data has increased by a factor of five, and the number of purified compounds with reference to NMR, LC-MS, and GC-MS spectra has more than doubled. In addition to this expansion, HMDB has introduced new searching tools, enhanced data content, and improved user interface for easier navigation and data retrieval. HMDB includes detailed information on over 6826 experimentally confirmed metabolites, representing a significant expansion of nearly 300% over the previous database. This increase is primarily due to the addition of over 3800 lipids detected and/or quantified in human tissues and biofluids. Other classes of compounds, such as glucuronides, carnitines, bile acids, and coenzyme A derivatives, have also seen substantial increases. The HMDB's content has been enhanced with more detailed compound descriptions, including more synonyms, doubling the number of compounds with NMR and MS spectra, and increasing the number of compounds with bio-fluid concentration data and synthesis records. HMDB has also introduced new data fields for MetaboCards, including chemical source information, physiological charge, experimental and predicted logP, HMDB pathway images, general metabolite references, and macromolecular interacting partners. The HMDB has also added new data fields for the BiGG database, Wikipedia, and METLIN, as well as extra data fields for GeneCard IDs, GeneAtlas IDs, and HGNC IDs for enzymes. The HMDB's user interface has been improved to accelerate searches, improve data visualization, and allow greater flexibility in the number of query tools and links. The HMDB's navigation bar has been simplified, and new browsing tools have been introduced, including HML Browse, ClassBrowse, PathBrowse, and Disease Browse. The HMDB's Search menu offers eight different querying tools, including ChemQuery, TextQuery, SequenceSearch, DataExtractor, MS search, MS-MS search, GC-MS search, and NMR search. These tools have been enhanced for speed, accuracy, and robustness. The HMDB's spectral databases and searching have also been improved, with comprehensive, publicly accessible libraries of reference compound spectra and robust search algorithms for spectral matching and compound identification. The HMDB's NMR data is fully assigned under standardized aqueThe Human Metabolome Database (HMDB) is a comprehensive, web-accessible resource that provides detailed information on human metabolites, their associated enzymes, transporters, abundance, and disease-related properties. Since its first release in 2007, HMDB has been used in nearly 100 published studies in metabolomics, clinical biochemistry, and systems biology. The latest version, HMDB 2.0, has significantly expanded and enhanced the database. The number of fully annotated metabolite entries has increased from 2180 to over 6800, a 300% increase. The number of metabolites with bio-fluid or tissue concentration data has increased by a factor of five, and the number of purified compounds with reference to NMR, LC-MS, and GC-MS spectra has more than doubled. In addition to this expansion, HMDB has introduced new searching tools, enhanced data content, and improved user interface for easier navigation and data retrieval. HMDB includes detailed information on over 6826 experimentally confirmed metabolites, representing a significant expansion of nearly 300% over the previous database. This increase is primarily due to the addition of over 3800 lipids detected and/or quantified in human tissues and biofluids. Other classes of compounds, such as glucuronides, carnitines, bile acids, and coenzyme A derivatives, have also seen substantial increases. The HMDB's content has been enhanced with more detailed compound descriptions, including more synonyms, doubling the number of compounds with NMR and MS spectra, and increasing the number of compounds with bio-fluid concentration data and synthesis records. HMDB has also introduced new data fields for MetaboCards, including chemical source information, physiological charge, experimental and predicted logP, HMDB pathway images, general metabolite references, and macromolecular interacting partners. The HMDB has also added new data fields for the BiGG database, Wikipedia, and METLIN, as well as extra data fields for GeneCard IDs, GeneAtlas IDs, and HGNC IDs for enzymes. The HMDB's user interface has been improved to accelerate searches, improve data visualization, and allow greater flexibility in the number of query tools and links. The HMDB's navigation bar has been simplified, and new browsing tools have been introduced, including HML Browse, ClassBrowse, PathBrowse, and Disease Browse. The HMDB's Search menu offers eight different querying tools, including ChemQuery, TextQuery, SequenceSearch, DataExtractor, MS search, MS-MS search, GC-MS search, and NMR search. These tools have been enhanced for speed, accuracy, and robustness. The HMDB's spectral databases and searching have also been improved, with comprehensive, publicly accessible libraries of reference compound spectra and robust search algorithms for spectral matching and compound identification. The HMDB's NMR data is fully assigned under standardized aque