The National Library of Medicine free article search engine, containing article references from MEDLINE, PreMEDLINE, OLDMEDLINE, plus some publisher-supplied and out of scope citations from certain general science and chemistry journals.
Scopus is an abstract and citation database of research literature and quality web sources, including journals, conference proceedings, trade publications, abstracts, and patent records. It is a multidisciplinary resource covering materials from the humanities, sciences and social sciences. Scopus allows you to locate the most highly cited items and the articles that cite them.
Date Coverage: 1996-present; selected access back to 1823