Integration of different information sources has been a problem that has been challenging (or perhaps better: plaguing) Computer Science throughout the decades. As soon as we had two computers, we wanted to exchange information between them, and as soon as we had two databases, we wanted to link them together. Fortunately, Computer Science has made much progress on different levels: Physical interoperability between systems has been all but solved: with the advent of hardware standards such as Ethernet, and with protocols such as TCP/IP and HTTP, we can nowadays walk into somebody's house or office, and successfully plug our computer into the network, giving instant world-wide physical connectivity. Physical connectivity is not sufficient. We must also agree on the syntactic form of the messages we will exchange. Again, much progress has been made in recent years, with open standards such HTML and XML.