Computer systems in clinical medicine are often integrated to facilitate data sharing, but the terminologies they use are typically not integrated. There is rarely any centralized repository of terms used in various systems, and no widely accepted standards exist. An exception is the Medical Entities Dictionary (MED) at the New York Presbyterian Hospital (NYPH) in New York. The NYPH clinical computing environment includes a centralized database of coded patient information collected from various ancillary sources and translated into codes provided by the MED. The MED includes all terms used in the ancillary departments and, as such, changes in the source terminologies must be maintained in the MED. In order to support the maintenance tasks, the MED includes knowledge about the terms themselves, arranged in a semantic network. This knowledge base is used to support sophisticated maintenance tools to facilitate additions and changes to the MED and verify their consistency and appropriateness. This chapter describes the design and development of the terminology and two case examples demonstrating some of the advantages to our approach: addition of a new terminology of laboratory terms and maintenance of an existing drug terminology.