Google, Yahoo!, and other companies vying for a piece of the lucrative search-engine market continue to transform the sponsored-search model, linking results to other information media such as telephones and TV. However, click fraud undermines this process by reducing the value of Web site traffic to content providers and thereby decreasing revenue for the search engines. Threats to the search engines' underlying business model are also threats to the free search services that these companies provide to millions of users, making click fraud a concern for all.