PubMed is a free database accessing primarily the MEDLINE

MEDLINE

MEDLINE is a bibliographic database of life sciences and biomedical information. It includes bibliographic information for articles from academic journals covering medicine, nursing, pharmacy, dentistry, veterinary medicine, and health care...

 database

Bibliographic database

A bibliographic database is a database of bibliographic records, an organized digital collection of references to published literature, including journal and newspaper articles, conference proceedings, reports, government and legal publications, patents, books, etc...

 of references and abstracts on life sciences and biomedical topics. The United States National Library of Medicine

United States National Library of Medicine

The United States National Library of Medicine , operated by the United States federal government, is the world's largest medical library. Located in Bethesda, Maryland, the NLM is a division of the National Institutes of Health...

 (NLM) at the National Institutes of Health

National Institutes of Health

The National Institutes of Health are an agency of the United States Department of Health and Human Services and are the primary agency of the United States government responsible for biomedical and health-related research. Its science and engineering counterpart is the National Science Foundation...

 maintains the database as part of the Entrez

Entrez

The Entrez Global Query Cross-Database Search System is a powerful federated search engine, or web portal that allows users to search many discrete health sciences databases at the National Center for Biotechnology Information website...

 information retrieval

Information retrieval

Information retrieval is the area of study concerned with searching for documents, for information within documents, and for metadata about documents, as well as that of searching structured storage, relational databases, and the World Wide Web...

 system. PubMed was first released in January 1996.
In addition to MEDLINE, PubMed provides access to:

• older references from the print version of Index Medicus
  Index medicus
  Index Medicus is a comprehensive index of medical scientific journal articles, published since 1879. It was initiated by John Shaw Billings, head of the Library of the Surgeon General's Office, United States Army...
  
  back to 1951 and earlier;
• references to some journals before they were indexed in Index Medicus and MEDLINE, for instance Science
  Science (journal)
  Science is the academic journal of the American Association for the Advancement of Science and is one of the world's top scientific journals....
  
  , BMJ
  BMJ
  BMJ is a partially open-access peer-reviewed medical journal. Originally called the British Medical Journal, the title was officially shortened to BMJ in 1988. The journal is published by the BMJ Group, a wholly owned subsidiary of the British Medical Association...
  
  , and Annals of Surgery
  Annals of Surgery
  The Annals of Surgery is a monthly review journal of surgical science and practice. It was started in 1885.-External links:** @ PubMed Central...
  
  ;
• very recent entries to records for an article before it is indexed with Medical Subject Headings
  Medical Subject Headings
  Medical Subject Headings is a comprehensive controlled vocabulary for the purpose of indexing journal articles and books in the life sciences; it can also serve as a thesaurus that facilitates searching...
  
   (MeSH) and added to MEDLINE; and
• a collection of books available full-text and other subsets of NLM records.

Many PubMed records contain links to full text articles, some of which are freely available, often in PubMed Central

PubMed Central

PubMed Central is a free digital database of full-text scientific literature in biomedical and life sciences. It grew from the online Entrez PubMed biomedical literature search system. PubMed Central was developed by the U.S. National Library of Medicine as an online archive of biomedical journal...

 and local mirrors such as UK PubMed Central

UK PubMed Central

' is an on-line database that offers free access to a vast and growing collection of biomedical and health research information.-Service:...

.

Information about the journals indexed in PubMed is found in the NLM Catalog.

, PubMed has over 21.2 million records going back to 1966, selectively to the year 1865, and very selectively to 1809; about 500,000 new records are added each year.