What If you are a large national advertising company with many different clients trying to reach millions of consumer? What if you were a large global manufacturer trying to reach potential consumers for your product? With millions of Web sites, working with each one would be impractical. Advertising networks solve this problem by creating a network of several thousand of the most popular Web sites visited by millions of people, tracking the behavior of these users across the entire network, building profiles of each user, and then selling these profiles to advertisers. Popular Web sites download dozens of Web tracking cookies, bugs, and beacons, which report user online behavior to remote servers without the user’s knowledge. Looking for young, single consumers, with college degrees, living in the Northeast, in the 18-34 age range who are interested purchasing a European car? Not a problem. Advertising networks can identify and deliver hundreds of thousands of people who fit this profile and expose them to ads for European cars as they move from one Web site to another. Estimates vary, but behaviorally targeted ads are 10 times more likely to produce a consumer response than a randomly chosen banner or video ad (see Figure 9.5). So-called advertising exchanges use this same technology to auction access to people with very specific profiles to advertisers in a few milliseconds. In 2011, about 20 percent of display ads are targeted; and the rest depend on the context of the pages shoppers visit or the estimated demographics of visitors.