A retrieval system containing citations, abstracts, and indexing terms for journal articles in the biomedical sciences. It includes literature citations supplied directly to NCBI by publishers as well as URLs to full text articles on the publishers' web sites. PubMed contains the complete contents of the MEDLINE and PREMEDLINE databases. It also contains some articles and journals considered out of scope for MEDLINE, based on either content or on a period of time when the journal was not indexed and, therefore, is a superset of MEDLINE.