Most syndromic surveillance systems use data from ED visits, often free-text chief complaints. Classification of chief complaints into syndromes is often inconsistent, due to the lack of agreement about the concepts that define a syndrome and how individual terms or strings map to these concepts. The Syndromic Surveillance Ontology addresses this problem by formally encoding a set of consensus definitions for syndromes in terms of ED chief complaints in a standard shareable format. The goal of the project is to ensure that consensus syndrome definitions are disseminated broadly, maintained collaboratively, and incorporated easily into automated systems.