Online ad verification startup DoubleVerify on Tuesday announced it has raised a cool $33 million in a third found of funding.

Companies like DoubleVerify and competitors like AdSafe and AdXpose serve an important role in the increasingly crowded — and uncertain — online ad marketplace. Publishers, marketers, ad networks and others in the ad supply chain use verification services to ensure that ads are being served correctly and to generally keep better track of individual ads.

“What sets us apart from our competition is our technology and our very high see-through rate,” Oren Netzer, CEO of DoubleVerify, told VentureBeat. “We can tell you 96 percent of the time where an ad was served. Our closest competitors only have see-through rates between 30 and 70 percent.”

DoubleVerifiy is based in New York, but also has offices in Tel Aviv and London, with about 160 employees total. Netzer said the new funding will go toward expanding the company by 100 employees over the next 12 months and adding more offices in Europe.

The company said its service is used by more than 200 of the leading Fortune 500 marketers. Its notable customers include American Express, Bank of America, Walmart, Wells Fargo, AT&T, Verizon and T-Mobile.

DoubleVerify’s new funding round was led by JMI Equity and Institutional Venture Partners. Other participants in the round included previous investors Blumberg Capital and First Round Capital.

“We had half a dozen offers, but JMI was the best choice for a new partner because they know the space so well,” Netzer said. “We want to be the next DoubleClick and JMI helped DoubleClick grow and eventually be sold to Google for $3.1 billion.”

Back in March 2010, the company raised $10 million in its second round and $3.5 million in its first round, with total funding reaching $46.5 million to date.