Online advertising has become a very profitable endeavor for publishers in the Interent. Its growth can be attributed to the rise in popularity of Google’s advertising method through search, and in fact, this industry will continue to rise annually by 12 percent in the coming 5 years. 2008 saw 23.4 billion dollars in overall sales in online advertising. This is the summary of a behavioral targeting article by Jonathan Levin and Paul Milgrom. You can get the pdf of the behavioral targeting article here: Online Advertising: Heterogeneity and Conflation in Market Design.