The Astrophysics Data System (ADS) is a NASA-funded project which maintains four bibliographic databases containing more than 4.4 million records: Astronomy and Astrophysics, Instrumentation, Physics and Geophysics, and preprints in Astronomy.

Integrated in its databases, the ADS provides access and pointers to a wealth of external resources, including electronic articles, data catalogs and archives. ADS currently has links to over 4.6 million records maintained by ADS collaborators.