The primary mission of web search engines is to obtain the best possible results for a given user query. To accomplish this effectively, they rely on two crucial pieces of information: the relevance of a web page to the query and some aspect of the quality of the web page that is independent of the query. Relevance, the extent to which the query matches the content of the web page, is formalized and extensively studied in the field of information retrieval. Quality, on the other hand, is more nebulous and less well-defined. Nevertheless, one can identify three concrete and somewhat complementary aspects to the quality of a web page. The first is based on the absolute goodness of the web page and its associated meta-data. This might depend on a variety of parameters, including the worth of content that exists on the web page, the reputation of the person who authored the web page, the importance of the web site that hosts the web page, and so on. The...