The two largest, general purpose data centres for astronomy world-wide are the ``Astronomical Data Center'' (ADC; adc.gsfc.nasa.gov/adc.html) of NASA's Astrophysics Data Facility (ADF), and the ``Centre de Données astronomiques de Strasbourg'' (CDS; cdsweb.u-strasbg.fr). They were the first institutions to systematically collect machine-readable versions of astronomical catalogues but have now widened their scope considerably. Several of their services are so diverse that they will be mentioned in different sections of this paper. At other, medium-sized, regional data centres (e.g. in Moscow, Tokyo, Beijing, Pune, etc.) some of the services of the two major centres are mirrored to reduce the network load on ADC and CDS.
The increasing number of space missions has led to the creation of mission-oriented data centres like:
and many others. Several major ground-based observatories have evolved data centres offering access to their archives of past observations. Examples are the European Southern Observatory (archive.eso.org), the Royal Greenwich Observatory Isaac Newton Group of Telescopes on La Palma (archive.ast.cam.ac.uk/ingarch), the National Radio Astronomy Observatory (NRAO) Very Large Array (www.aoc.nrao.edu/vla/html/VLAhome.shtml), the Westerbork Synthesis Radio Telescope (WSRT; www.nfra.nl/scissor), etc.
Many bulky data sets, like e.g. the IRAS data products, the Hubble Space Telescope (HST) Guide Star Catalogue (GSC), etc., have been widely distributed on CD-ROMs. For a comprehensive, though not complete, list of CD-ROMs in astronomy see the ``Mediatheque'' maintained at CDS (cdsweb.u-strasbg.fr/mediatheque) and D. Golombek's contribution to these proceedings.