Previous investigators have defined clinical interface terminology as a systematic collection of health care-related phrases (terms) that supports clinicians' entry of patient-related information into computer programs, such as clinical "note capture" and decision support tools. Interface terminologies also can facilitate display of computer-stored patient information to clinician-users. Interface terminologies "interface" between clinicians' own unfettered, colloquial conceptualizations of patient descriptors and the more structured, coded internal data elements used by specific health care application programs. The intended uses of a terminology determine its conceptual underpinnings, structure, and content. As a result, the desiderata for interface terminologies differ from desiderata for health care-related terminologies used for storage (e.g., SNOMED-CT), information retrieval (e.g., MeSH), and classification (e.g., ICD9-CM). Necessary but not sufficient attributes for an interface terminology include adequate synonym coverage, presence of relevant assertional knowledge, and a balance between pre- and post-coordination. To place interface terminologies in context, this article reviews historical goals and challenges of clinical terminology development in general and then focuses on the unique features of interface terminologies.