BRENDA (The Comprehensive Enzyme Information System) is an information system representing one of the most comprehensive enzyme repositories. It is an electronic resource that comprises molecular and biochemical information on enzymes that have been classified by the IUBMB. Every classified enzyme is characterized with respect to its catalyzed biochemical reaction. Kinetic properties of the corresponding reactants (that is, substrates and products) are described in detail. BRENDA contains enzyme-specific data manually extracted from primary scientific literature and additional data derived from automatic information retrieval methods such as text mining. It provides a web-based user interface that allows a convenient and sophisticated access to the data. History BRENDA was founded in 1987 at the former German Research Centre for Biotechnology (now the Helmholtz Centre for Infection Research) in Braunschweig and was originally published as a series of books. Its name was originally an acronym for the BRaunschweig ENzyme DAtabase. From 1996 to 2007, BRENDA was located at the University of Cologne. There, BRENDA developed into a publicly accessible enzyme information system. In 2007, BRENDA returned to Braunschweig. Currently, BRENDA is maintained and further developed at the BRICS ( Braunschweig Integrated Centre of Systems Biology) at the TU Braunschweig. In 2018 BRENDA was appointed to ELIXIR Core Data Resource with fundamental importance to biological and biomedical research and long-term preservation of biological data. Updates A major update of the data in BRENDA is performed twice a year. Besides the upgrade of its content, improvements of the user interface are also incorporated into the BRENDA database. Content and features Database: The database contains more than 40 data fields with enzyme-specific information on more than 8300 EC numbers that are classified according to the IUBMB. The different data fields cover information on the enzyme's nomenclature, reaction and specificity, enzyme structure, isolation and preparation, enzyme stability, kinetic parameters such as Km value and turnover number, occurrence and localization, mutants and engineered enzymes, application of enzymes and ligand-related data. Currently, BRENDA contains manually annotated data from over 165,000 different scientific articles. Each enzyme entry is clearly linked to at least one literature reference, to its source organism, and, where available, to the protein sequence of the enzyme. An in-house development of the BRENDA team is the BRENDA tissue ontology (BTO), a comprehensive, structured encyclopedia with controlled vocabulary for terms and names for tissues, organs, anatomical structures, plant parts, cell cultures, cell types, and cell lines in organisms