PubMed is a free resource developed and maintained by the National Center for Biotechnology Information at the U.S. National Library of Medicine, located at the National Institutes of Health. It comprises over 22 million citations for biomedical literature from MEDLINE, life science journals, and online books. PubMed citations and abstracts include the fields of biomedicine and health, covering portions of the life sciences, behavioral sciences, chemical sciences, and bioengineering. 

For further information, see the extensive PubMed Help section.