MetaMap is a highly configurable application developed by the Lister Hill National Center for Biomedical Communications at the National Library of Medicine (NLM) to map biomedical text to the UMLS Metathesaurus or, equivalently, to identify Metathesaurus concepts referred to in English text. MetaMap employs a knowledge-intensive approach, natural-language processing (NLP), and computational-linguistic techniques, and is used worldwide in industry and academia. At NLM, MetaMap is one of the foundations of NLM's Medical Text Indexer (MTI), which is applied to both semiautomatic and fully automatic indexing of biomedical literature.