The Astrophysics Data System is an online NASA database of over 7,000,000 astronomy and physics papers from both peer reviewed and non-peer reviewed sources. Abstracts are available free online for almost all articles, and full scanned articles are available in GIF and PDF format for older articles. New articles have links to electronic versions hosted at the journal's webpage, but these are typically available only by subscription (which most astronomy research facilities have). It is managed by the Harvard-Smithsonian Center for Astrophysics. ADS is an extremely powerful research tool, and has had a significant impact on the efficiency of astronomical research since it was launched in 1992. Literature searches which previously would have taken days or weeks can now be carried out in seconds via the sophisticated ADS search engine, which is custom-built for astronomical needs. Studies have found that the benefit to astronomy of the ADS is equivalent to several hundred million US dollars annually, and the system is estimated to have tripled the readership of astronomical journals. Use of ADS is almost universal among astronomers worldwide, and therefore ADS usage statistics can be used to analyze global trends in astronomical research.