RANKED: The Hottest Pre-IPO Adtech Startups Of 2014

Robert Libetti/ Business InsiderAppNexus CEO Brian O'Kelley

Last year, we began ranking pre-IPO adtech companies by size and importance. Since then, the adtech IPO market has just gotten hotter.

Rubicon Project filed for an IPO, and its stock popped nicely on the first day - making adtech IPOs suddenly much more interesting than they used to be.

Tubemogul also filed for an IPO. DataXu confirmed it will do the same thing.

And, biggest of all, Pinterest looks like it is gearing up an adtech revenue model - always a pre-IPO sign.

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).

This ranking looks at the hotter companies subject to IPO (or at least big exit deal) rumors right now. Our data comes from Crunchbase, LinkedIn, our own reporting and that of other business publications. We've tried to rank them by revenues. See our methodology at the end.