‘Pantha rhei,’ was one of the tenets of the Greek philosopher Heraclitus. This observation on life in Greece in the sixth century BC is certainly relevant to the World Wide Web some 2,500 years later. On the Web everything is in a state of flux and is subject to continuous change. The Web is expanding at an enormous pace; millions of new pages appear every month. The number of sites - the building blocks of the Web - is increasing by millions each year and may have amounted to 162 million in July 2002 (Internet Software Consortium, www.isc.org/ds/WWW-200207/index.html). This incredible expansion is taking place despite the fact that at the same time as new sites are created, many disappear: the aveage life span of a site is estimated at seventy-five days. Sites are often provided at only one location. If, for whatever reason, a supplier decides to discontinue a site, it is lost forever. In this dynamic process of rise and fall most existing sites are not static either.