RANKED: The 15 Hottest Pre-IPO Adtech Startups Right Now

The adtech business is heating up in terms of IPOs and acquisitions. Half a dozen adtech companies have staged public offerings recently, with varying levels of success.

In the last year or so we've seen IPOs from Millennial Media, which raised $130 million, Tremor Video ($75 million), YuMe ($46 million), Rocket Fuel ($116 million), Marin Software ($105 million) and Criteo ($190 million).

On top of that, the sector is ripe for buyouts: Twitter just acquired MoPub for $350 million in stock. AOL acquired Adap.tv for $405 million.

Rumors are circulating heavily that AppNexus, the big New York ad server which serves both buyers and sellers, will stage an IPO sometime soon. Mobile ad company Flurry has said it will eventually be required to do so.

We decided to look at some of the hotter companies subject to IPO (or at least big exit deal) rumors right now. We're not saying all these companies will file IPOs. But we are saying they're ripe for "liquidity events" (big financial deals) of one sort or another.

Our data comes from Crunchbase, LinkedIn, our own reporting and that of other business publications. We've ranked the companies by employee headcount.