The purpose of the National Library of Medicine Unified Medical Language System (UMLS) is to facilitate the development of computer systems that behave as if they "understand" the meaning of the language of biomedicine and health. The UMLS provides data for system developers as well as search and report functions for less technical users.

There are three UMLS Knowledge Sources:

• The Metathesaurus, which contains over one million biomedical concepts from over 100 source vocabularies
• The Semantic Network, which defines 133 broad categories and fifty-four relationships between categories for labeling the biomedical domain
• The SPECIALIST Lexicon & Lexical Tools, which provide lexical information and programs for language processing

They are distributed with flexible lexical tools and MetamorphoSys, the UMLS install and customization program.