The Internet has grown dramatically over the past few years.
Information sources are available everywhere.
Unfortunately, these information sources vary widely in the types of information and access interfaces that they provide. Therefore, using this wealth of resources effectively presents interesting and challenging problems. In effect, users have information needs, and should not be concerned with the format of the available data, or with the interface and access capabilities of the data sources. Increasingly, users want to issue complex queries across Internet sources to obtain the data they require. Because of the size of the Internet, it is not possible anymore to process such queries in naive ways, e.g., by accessing all the available sources.