Recommender systems give advice about products, information or services users might be interested in. They are intelligent applications to assist users in a decision-making process where they want to choose one item amongst a potentially overwhelming set of alternative products or services. And they are probably among the most prominent applications having a substantial impact on the performance of e-commerce sites and the sector in general. In fact, even if the problem of supporting a choice decision process is quite old, it is only with the advent of the WWW that we had at disposal, in a large quantity, two basic ingredients of recommender systems: a large catalogue of products, as in e-commerce web sites, and a large population of consumers/users querying these web sites and leaving track of they on-line behavior. From their introduction, recommender systems have been exploited for recommending books, CDs, movies, news, electronics, travels, financial services, and many other products and services. Recommender systems are becoming more and more popular even in rather simple e-commerce web sites.