The Smithsonian/NASA Astrophysics Data System (ADS) is a Digital Library portal for researchers in Astronomy and Physics, currently being developed by the Smithsonian Astrophysical Observatory under a NASA grant. The ADS maintains three bibliographic databases containing more than 7.5 million bibliographic records covering the scholarly literature in Astronomy and Physics.

For each bibliographic record in its database, the ADS maintains a list of links to relevant resources, including:

    * Location of the fulltext article via DOI and/or OpenURL links
    * List of works referenced in the original article (references), given in the form of either a list of ADS records or as a link to the publisher’s reference list
    * List of works that cite the record in question (citations or “forward links”), given in the form of a list of records that ADS was able successfully identify
    * Readership statistics and co-readership-based user recommendations

The ADS also maintains an archive of the historical content of all the astronomical publications.  The contents of the archive consist of 500,000 articles, corresponding to over 3.5 million scanned pages, which have been OCRed and which are searchable through the ADS fulltext query search form.