Breakthroughs in telescope, detector and computer technology now allow astronomical surveys to produce large numbers of images, spectra and catalogues. These data sets cover the sky at all wavelengths, from gamma- and X-rays through optical, infrared and radio waves. Astronomers are developing ways to make the large amount of data easily accessible. These techniques use the grid paradigm of distributed computing with seamless, transparent access to data through virtual observatories (VOs). As a physical observatory has telescopes with unique astronomical instruments, a VO consists of data centres with unique collections of astronomical data, software systems and processing capabilities. This global, community-based initiative is being developed under the International Virtual Observatory Alliance and in Europe as part of the EURO-VO project.