During the past decade our society has experienced a revolutionary increase in the extent to which the general public has access to electronic resources. This phenomenon has been characterised by: International trends leading to unification. Increased availability of personal computers at home in all developed countries. Wide-spread use of electronic media in our daily life, and not just in the work environment: cash dispensers, cinema and theatre tickets, computer games, electronic mail, ... Diversity of multimedia products: encyclopaedia and dictionaries, cook books, children's books, etc. Specific environments dominate the market: WWW browsers, notably Netscape and Microsoft Explorer, have effectively become standard interfaces...