Reactome is a free, online, curated database of core pathways and reactions in human biology that functions as a data mining resource and electronic textbook. Information is authored by expert biological researchers, maintained by the Reactome editorial staff and cross-referenced to NCBI Entrez Gene, Ensembl and UniProt databases, the UCSC and HapMap Genome Browsers, the KEGG Compound and ChEBI small molecule databases, PubMed, and GO. Its current release (Version 29, July 2009) includes 4181 human proteins, 3335 reactions and 990 pathways. An entity-level pathway viewer and data mining tools facilitate searching and visualizing pathway data and the analysis of user-supplied high-throughput data sets. Reactome has increased its utility to the model organism communities with improved orthology prediction methods allowing pathway inference for 22 species and through collaborations to create manually curated Reactome pathway datasets for species including Arabidopsis, Oryza sativa (rice), Drosophila and Gallus gallus (chicken). Reactome’s data content and software can all be freely used and redistributed under open source terms.