PubMed is a free database accessing the MEDLINE database of citations, abstracts and some full text articles on life sciences and biomedical topics. The United States National Library of Medicine (NLM) at the National Institutes of Health (NIH) maintains PubMed as part of the Entrez information retrieval system. Listing an article or journal in PubMed is not endorsement. In addition to MEDLINE, PubMed also offers access to

• OLDMEDLINE for pre-1966 citations. This has recently been enhanced, and records for 1951+, even those parts in the printed indexes, are now included within the main portion.
• Citations to all articles (even those that are out-of-scope, e.g., covering plate tectonics or astrophysics) from certain MEDLINE journals, primarily the most important general science and chemistry journals, from which the life sciences articles are indexed for MEDLINE.
• In-process citations which provide a record for an article before it is indexed with MeSH and added to MEDLINE or converted to out-of-scope status.
• Citations that precede the date that a journal was selected for MEDLINE indexing (when supplied electronically by the publisher).
• Some life science journals that submit full text to the PubMed Central digital library and may not have been recommended for inclusion in MEDLINE although they have undergone a review by NLM, and some physics journals that were part of a prototype PubMed in the early to mid-1990s.[1]

Many PubMed citations contain links to full text articles which are freely available, often in PubMed Central. In late 2007, President George W. Bush signed the Consolidated Appropriations Act of 2007 (H.R. 2764) into law; this law included a provision requiring the NIH to modify its policies and require inclusion into PubMed Central complete electronic copies of their peer-reviewed research and findings from its funded research. This is the first time the US government has required an agency to provide open access to research and is an evolution from the 2005 policy, in which the NIH asked researchers to voluntarily add their research to PubMed Central. With an effective date of 7 April 2008, the Department of Health and Human Services gave notice: "The Director of the National Institutes of Health shall require that all investigators funded by the NIH submit or have submitted for them to the National Library of Medicine's PubMed Central an electronic version of their final, peer-reviewed manuscripts upon acceptance for publication, to be made publicly available no later than 12 months after the official date of publication: Provided, That the NIH shall implement the public access policy in a manner consistent with copyright law".

The National Library of Medicine also leases the MEDLINE information to a number of private vendors such as Ovid and SilverPlatter – as well as many other vendors. PubMed has been available free on the Internet since the mid-1990s.

Information about the journals indexed in PubMed is found in its Journals Database, searchable by subject or journal title, Title Abbreviation, the NLM ID (NLM's unique journal identifier), the ISO abbreviation, and both the print and electronic International Standard Serial Numbers (pISSN and eISSN). The database includes all journals in all Entrez databases.

As of 29 July 2010 (2010 -07-29)^[update], PubMed has approximately 20 million citations going back to the year 1865. To see the current size of the database simply type "1800:2100[dp]" into the search bar and click "search".