  The NASA Astrophysics Data System Abstract Service has become a key component
of astronomical research. It provides bibliographic information daily, or near
daily, to a majority of astronomical researchers worldwide.
  We describe the history of the development of the system and its current
status.
  We show several examples of how to use the ADS, and we show how ADS use has
increased as a function of time. Currently it is still increasing
exponentially, with a doubling time for number of queries of 17 months.
  Using the ADS logs we make the first detailed model of how scientific
journals are read as a function of time since publication.
  The impact of the ADS on astronomy can be calculated after making some simple
assumptions. We find that the ADS increases the efficiency of astronomical
research by 333 Full Time Equivalent (2000 hour) research years per year, and
that the value of the early development of the ADS for astronomy, compared with
waiting for mature technologies to be adopted, is 2332 FTE research years.
  The ADS is available at http://adswww.harvard.edu/.
