Personalized information filtering by online search engines, social media, news sites and retailers represents a natural evolution in the development towards ever more finely tuned interaction with the users. Since the internet provides an overwhelming quantity of information on most topics, information overload has become one of the main concerns for users. Perceived quality of information services is therefore strongly determined by the ease with which the user can obtain some information that satisfies their current desires. For many of the most highly success internet service, like Google, Amazon.com, YouTube, Netflix and TripAdvisor, the recommender system is a key element in their success over rival services in the same sector. Some, like Netflix, openly acknowledge this even to the extent of awarding large prizes for anyone that can improve their recommender system.