Reactome is a curated, peer-reviewed resource that integrates and presents biological processes in humans. The primary unit of the Reactome database is a reaction, which can be a biochemical reaction or a less conventional type of event. Reactions are grouped into pathways, which can be sequential, parallel, or ordered in a cycle. Reactome provides a qualitative framework that can be overlaid with quantitative data, facilitating the analysis of high-throughput data. The database covers a wide range of biological processes, including the cell cycle, DNA repair, gene expression, metabolism, and signal transduction. Reactome is publicly available under open-source terms, allowing both content and software to be freely used and redistributed. The project uses a combination of curated data from human experiments and electronic projections from other organisms via putative orthologs. The Reactome user interface includes a Reaction Map, which visually represents all reactions in the database, and a 'skypainter' tool that allows users to colorize the map based on gene or protein identifiers. The data model and acquisition process involve a rigorous curation process, with authors creating modules using the Reactome Author Tool and curators refining the annotations. Reactome aims to provide a comprehensive and integrated view of biological processes, supporting both individual researchers and bioinformaticians.Reactome is a curated, peer-reviewed resource that integrates and presents biological processes in humans. The primary unit of the Reactome database is a reaction, which can be a biochemical reaction or a less conventional type of event. Reactions are grouped into pathways, which can be sequential, parallel, or ordered in a cycle. Reactome provides a qualitative framework that can be overlaid with quantitative data, facilitating the analysis of high-throughput data. The database covers a wide range of biological processes, including the cell cycle, DNA repair, gene expression, metabolism, and signal transduction. Reactome is publicly available under open-source terms, allowing both content and software to be freely used and redistributed. The project uses a combination of curated data from human experiments and electronic projections from other organisms via putative orthologs. The Reactome user interface includes a Reaction Map, which visually represents all reactions in the database, and a 'skypainter' tool that allows users to colorize the map based on gene or protein identifiers. The data model and acquisition process involve a rigorous curation process, with authors creating modules using the Reactome Author Tool and curators refining the annotations. Reactome aims to provide a comprehensive and integrated view of biological processes, supporting both individual researchers and bioinformaticians.