The recent surge of interest in information filtering has actually contributed to the flood of information, since there is now more being published in the field than any single individual could hope to read. In part this results from the coincident adoption of the World Wide Web as a rapid means for the dissemination of academic work. Presently there are literally hundreds of documents about information filtering accessible through that medium. In this section we describe the two dominant research paradigms, content-based and social filtering, and examine issues related to each. We have selected systems to discuss which highlight the most important approaches that have been used and the significant issues which have been raised.