This article describes the World Wide Web (WWW) Entrez server which is based upon the National Center for Biotechnology Information's (NCBI) Entrez retrieval database and software. Entrez is a molecular sequence retrieval system that contains an integrated view of portions of Medline and all publicly available nucleotide and protein databases, including GenBank. A CD-ROM is available but the WWW server enables those who access the databases to have enhanced capabilities. Sections relating to the history of Entrez and WWW Entrez, a comparison of three types of Entrez, and a discussion of conclusions and future directions are included. (DDR)