PubMed is a free online database of biomedical and life sciences literature. It contains over 30 million citations from MEDLINE, the U.S. National Library of Medicine's (NLM) premier bibliographic database, as well as other sources. PubMed includes links to full-text articles and other resources.
2. ScienceDirect
ScienceDirect is a subscription-based online database of scientific, technical, and medical (STM) literature. It contains over 40 million articles from over 3,500 journals and over 34,000 books. ScienceDirect includes links to full-text articles, book chapters, and other resources.
3. JSTOR
JSTOR is a digital library of academic journals, books, and other primary sources. It contains over 12 million pages from over 1,000 journals and over 5,000 books. JSTOR includes links to full-text articles, book chapters, and other resources.