The NCI Thesaurus is a public domain description logic-based terminology produced by the National Cancer Institute. The NCI Thesaurus is used to support storage and retrieval of scientific, clinical and research administration data. The content of the NCI Thesaurus evolves rapidly. We have developed a representation of concept change over time and have implemented software to capture concept change in our multi-editor concurrent vocabulary development environment. We are now implementing software to extend our vocabulary server, public APIs and our file-based distributions of the Thesaurus to provide access to the concept-level history information.