A  look at the future of advertising:

Advertisers no longer want to just buy ads. They want to buy access to specific people. So, Mr. Norris is building a "credit bureau for devices" in which every computer or cellphone will have a "reputation" based on its user's online behavior, shopping habits and demographics. He plans to sell this information to advertisers willing to pay top dollar for granular data about people's interests and activities.

Device fingerprinting is a powerful emerging tool in this trade. It's "the next generation of online advertising," Mr. Norris says.

It might seem that one computer is pretty much like any other. Far from it: Each has a different clock setting, different fonts, different software and many other characteristics that make it unique.  Every time a typical computer goes online, it broadcasts hundreds of such details as a calling card to other computers it communicates with. Tracking companies can use this data to uniquely identify computers, cellphones and other devices, and then build profiles of the people who use them.

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