The Epidemiology Ontology is an ontology designed to support the semantic annotation of epidemiology resources. It is being developed under the EU-funded EPIWORK project, a multidisciplinary research effort which aims at increasing the amount of epidemiological data available, improving disease surveillance systems, and promoting the collaboration among epidemiological researchers. The EO is integrated into NERO (Network of Epidemiology Related Ontologies), a collection of existing ontologies that supports the semantic annotation of epidemiology resources contained in the Epidemic Marketplace (EM), a platform for sharing resources and knowledge within the Epidemiology community. NERO currently includes thirteen external ontologies (the majority are OBO or OBO candidate ontologies) which already provide a high coverage of most epidemiology related areas. As such, the EPO focuses on neglected/highly specific areas of epidemiology and will articulate with other OBO ontologies as needed.