In 2003, the Department of Veterans Affairs Veterans Health Administration (VHA) began construction of an Enterprise Reference Terminology (ERT)1. ERT includes a federated set of vocabulary content; a hybrid commercial off-the-shelf (COTS) and custom-developed software and technology infrastructure; and supporting business processes and documentation. Since its inception, VHA ERT assembled one of the largest terminology repositories in the country. Controlled medical terminology provides many benefits, and chief among them is support for the creation and use of comparable patient descriptions. Such data comparability can help: Reduce ambiguity while describing medical situations Improve human productivity Improve the performance of decision support applications Improve compliance with existing or emerging VHA and other federal mandates and standards Enable the exchange of healthcare information o between departments in the same VHA medical center or care facility o between VHA and extra-VHA facilities o between applications Manage and leverage information in electronic medical records Improve the display of patient information Make CPOE (Computer-based Provider Order Entry) more productive Enable decision support to reduce errors and improve quality Support evidence-based medicine Use of controlled terminology in electronic medical records greatly enhances the ability of both healthcare professionals and computer applications to collect and leverage available healthcare data productively. The VHA ERT is designed to provide terminology and terminology services that support these objectives at national scale. In this context, the notion of a reference terminology is a resource focused on scalable, longitudinal terminology reuse by computers, applications, and their human users.