The ability of storing, managing and giving access to the huge quantity of data collected by astronomical observatories is one of the major challenges of modern astronomy. At the same time, the growing complexity of data systems implies a change of concepts: the scientist has to manipulate data as well as information. Developments of the "World Wide Web" bring answers to these problems. The book presents a wide selection of databases, archives, data centres and information systems. Descriptions are included, together with their scientific context and motivations. This volume should prove a useful tool for astronomers, librarians, data specialists and computer engineers.