Reactome is a curated database of biological processes in humans. It covers biological pathways ranging from the basic processes of metabolism to high-level processes such as hormonal signalling. While Reactome is targeted at human pathways, it also includes many individual biochemical reactions from non-human systems such as rat, mouse, fugu fish and zebra fish. This makes the database relevant to the large number of researchers who work on model organisms. All the information in Reactome is backed up by its provenance: either a literature citation or an electronic inference based on sequence similarity. Our ontology ensures that the various events are linked in an appropriate spatial and temporal context.