Reactome is a curated, peer-reviewed knowledgebase of human biological pathways. It provides a qualitative framework for biological processes, allowing quantitative data to be superimposed. The database is publicly available under open source terms, enabling free use and redistribution of its content and software. The basic unit of Reactome is a reaction, which is any event converting inputs to outputs. Reactions are grouped into pathways, which can be sequential, parallel, or cyclic. Pathways can also nest, containing other pathways as components. Reactome includes pathways for fundamental biological processes such as the cell cycle, DNA repair, gene expression, and metabolism. It also includes non-human pathways, which are inferred based on putative orthologs. The database is used by researchers to explore biological processes, annotate genes and proteins, and visualize pathways. Reactome provides tools for custom data entry, annotation, and visualization of pathways as interactive process maps. Reactome is organized like an online journal, with modules authored by biologists and curated by full-time staff. Modules are reviewed by peer reviewers and then made available to the public. The database is continuously updated through rolling reviews and new data integration. Reactome supports data exchange with other pathway and molecular interaction databases through standard formats such as BioPAX, SBML, and PSI-MI. Reactome provides a biologist-friendly visualization of biological pathways and is an open-source project. It serves as a framework for quantitative models of physiology when combined with expression and enzyme kinetic data. The next data release will include pathways related to apoptosis, hemostasis, and other topics. Reactome is a valuable resource for both students and bioinformaticians, offering high-quality curated summaries of fundamental biological processes in humans.Reactome is a curated, peer-reviewed knowledgebase of human biological pathways. It provides a qualitative framework for biological processes, allowing quantitative data to be superimposed. The database is publicly available under open source terms, enabling free use and redistribution of its content and software. The basic unit of Reactome is a reaction, which is any event converting inputs to outputs. Reactions are grouped into pathways, which can be sequential, parallel, or cyclic. Pathways can also nest, containing other pathways as components. Reactome includes pathways for fundamental biological processes such as the cell cycle, DNA repair, gene expression, and metabolism. It also includes non-human pathways, which are inferred based on putative orthologs. The database is used by researchers to explore biological processes, annotate genes and proteins, and visualize pathways. Reactome provides tools for custom data entry, annotation, and visualization of pathways as interactive process maps. Reactome is organized like an online journal, with modules authored by biologists and curated by full-time staff. Modules are reviewed by peer reviewers and then made available to the public. The database is continuously updated through rolling reviews and new data integration. Reactome supports data exchange with other pathway and molecular interaction databases through standard formats such as BioPAX, SBML, and PSI-MI. Reactome provides a biologist-friendly visualization of biological pathways and is an open-source project. It serves as a framework for quantitative models of physiology when combined with expression and enzyme kinetic data. The next data release will include pathways related to apoptosis, hemostasis, and other topics. Reactome is a valuable resource for both students and bioinformaticians, offering high-quality curated summaries of fundamental biological processes in humans.