Cookies are the basis for billions of dollars of online advertising. They’re also under fire from privacy advocates, the government, and even makers of Web browsers. But the key problem is the fragmentation of how people access online content (from phone to computer to tablet and back again).

And so the worries about how advertisers will be able to reach fragmented, increasingly privacy-aware audiences continue to consume the online ad business. Problem is, they haven’t yet come up with solutions–and the clock is ticking.