In the past 50 years, the number of telescopes worldwide has exploded, creating at the same time vast quantities of information. This has boosted research considerably and increased knowledge of our place in the Universe. But much more remains to be discovered, if only astronomers had access to all the possible data available on celestial objects. An astronomer may have access to images from NASA, but can he or she compare them to those acquired by the European Southern Observatory, or the European Space Agency? Do they have access to all the observations taken by every available telescope, on the ground and in space? Virtual observatories (VOs) may provide the answer, making vast quantities of astronomical data available over the internet.