PubMed (NLM)

Over 20 million citations for biomedical literature from MEDLINE, National Center for Biotechnology Information, Genome Project and other sources, some with links to full-text content; also includes MeSH – the database of standardized subject headings, PubMed Central – the NIH free digital archive of journal literature, clinical trials, tutorials, etc. Coverage varies by source; Medline citations go back to 1966.