The Unified Medical Language System (UMLS) project is along-term R&D effort with the ambitious goal of enabling computer systems to understandÓ medical meaning. This is essential to the development of advanced health information systems. This annotated bibliography of 280 citations covers: UMLS knowledge sources; UMLS applications (vocabulary construction & concept discovery; data creation; natural language processing, indexing, & retrieval; linking clinical systems to knowledge-based info. sources; access to multiple knowledge-based info. sources); preliminary & ancillary studies; UMLS in relation to other programs; & opinions about UMLS.