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The recommendation of products, content and services cannot be considered newly born, although its widespread application is still in full swing. While its growing success in numerous sectors, the progress of the Social Web has revolutionized the architecture of participation and relationship in the Web, making it necessary to restate recommendation and reconciling it with Collaborative Tagging, as the popularization of authoring in the Web, and Social Networking, as the translation of personal relationships to the Web. Precisely, the convergence of recommendation with the above Social Web pillars is what motivates this book, which has collected contributions from well-known experts in the academy and the industry to provide a broader view of the problems that Social Recommenders might face with. If recommender systems have proven their key role in facilitating the user access to resources on the Web, when sharing resources has become social, it is natural for recommendation strategies in the Social Web era take into account theusers'point of view and the relationships among users to calculate their predictions. This book aims to help readers to discover and understand the interplay among legal issues such as privacy; technical aspects such as interoperability and scalability; and social aspects such as the influence of affinity, trust, reputation and likeness, when the goal is to offer recommendations that are truly useful to both the user and the provider.

The Portuguese Association for Arti cial Intelligence (APPIA) has been re- larly organising the Portuguese Conference on Arti cial Intelligence (EPIA). This ninth conference follows previous ones held in Porto (1985), Lisboa (1986), Braga (1987), Lisboa (1989), Albufeira (1991), Porto (1993), Funchal (1995) and Coimbra (1997). Starting in 1989, the conferences have been held biennially (alternating with an APPIA Advanced School on Arti cial Intelligence) and become truly international: English has been adopted as the o cial language and the proceedings are published in Springer's LNAI series. The conference has recon rmed its high international standard this year, largely due to its programme committee, composed of distinguished researchers in a variety of specialities in Arti cial Intelligence, half of them from Portuguese universities. This has attracted a signi cant international interest, well expressed by the number of papers submitted (66), from 17 di erent countries, 29 of which are by Portuguese researchers. From the 66 papers submitted, about one third of them (23) were selected for oral presentation and have been published in this volume. The review process enabled the selection of high quality papers, each paper being reviewed by two or three reviewers, either from the programme committee or by their appointment. We would like to thank all of the reviewers for their excellent and hard work.