The behaviour of pervasive molecular processes in human biology can be studied through the large-scale modeling of the molecular events that define them. Constructing detailed models of such extent and scope is a considerable undertaking well beyond the reach and capability of individual efforts, due to the range of expertise required. Reactome (http://www.reactome.org) is an open-access project that collaborates with field experts to integrate their pathway knowledge into a single quality-checked human model. This resource dataset is systematically cross-referenced to major molecular and literature databases, and is accessible to the community in a number of well-established formats. Various tools have been developed to facilitate querying and interaction with this content. The salient features of the annotation strategy are discussed here, and examples of pathway and genomic data integration using flexible interfacing methods from the associated toolkit are also presented.