Traditional database and information retrieval systems have followed a query-based information access paradigm (i.e., information is returned to the user on the basis of a query issued). As a result, users issuing the same query are provided with the same answer. With the advent of the World Wide Web and hand-held electronic devices such as palmtops and cellular phones, information access entered a new era. Increasing amounts of information become available to a growing mass of untrained lay users through various access media. A user searching Web-resident information may have to reformulate queries issued several times and sift through many results until a satisfactory, if any, answer is obtained. As purely query-driven approaches may be inappropriate in this context, the need for a shift towards a more user-centered information access paradigm arises. To this end, different approaches aim to the personalization of the overall user experience at different levels: content selection, content presentation, and user interaction.