The PubMed database contains more than 30 million citations and abstracts of biomedical literature. PubMed was developed and is maintained by the National Center for Biotechnology Information (NCBI) , at the U.S. National Library of Medicine (NLM) , located at the National Institutes of Health (NIH). It facilitates searching across several NLM resources, including MEDLINE, PubMed Central, and Bookshelf. A well-known medical resource, PubMed's resources are extensive, authoritative, and offer complex searching options. Coverage in this database extends from the 1940s to the present day.