Research in the Koninklijke Bibliotheek (KB) of the Netherlands in the 1990s eventually led to the 80 computer workstations now available for its registered users, allowing access to a multitude of catalogues both in the library and on the Internet. A system called KB-CAT has been developed that allows users to search multiple databases with one query. Three design principles underlie the system: first, for security, KB-CAT never searches directly in the original databases, but always in a copy; second, a simple, extendable, unifying MARC-like format encompassing all databases has been created; third, a pragmatic approach has been taken to deal with semantic interoperability. The architecture of KB-CAT has two separate parts: a system-oriented back end (a database called KBTITLE, which holds copies of records from other databases); and a user- or product-oriented front end where the user services (e.g. search and retrieval) are implemented. Twenty-one catalogues are already available on KB-CAT, and new digital services are frequently accommodated, most recently Netuit, a service aimed at the book trade, which lists all recently published books in the Netherlands, and Book History Online, a global annual bibliography of the history of the printed book.