Serial, a spinoff of the popular This American Life podcast, debuted in October 2014 to little fanfare but would go on to become the digital medium's first monster hit. Listeners became enthralled by host Sarah Koenig's deep dive into the 1999 true-crime case of Adnan Syed. During its debut run, Serial garnered more than 100 million downloads on iTunes, averaging around 8.3 million per episode.

"If there is a renaissance in podcasting, then it's because of This American Life, Serial, [This American Life host] Ira Glass and [Serial producer] Sarah Koenig," said Mark McCrery, CEO and founder of Podtrac, which handles advertising for Serial and This American Life, as well as WNYC's Freakonomics and Psychobabble with Tyler Oakley.

Serial's Season 2, which reportedly will focus on ex-POW Sgt. Bowe Bergdahl who was held captive by the Taliban in Afghanistan for five years, is expected to launch in the coming weeks. The series will be available on iTunes and other podcast apps, as well as Pandora (Season 1 is there now). Email marketing service MailChimp was the show's first sponsor, with Squarespace and Amazon's Audible coming aboard midway through the season. All three quickly renewed their commitment and will be back as launch sponsors, with bigger brands coming in between 10 and 30 days after launch. Pandora has also signed Warner Bros., which will promote upcoming film In the Heart of the Sea, and Esurance to sponsor its stream of Serial.

"The sheer scale of its listener base—tens of millions—can be attributed as a key catalyst for the medium to be considered in planning discussions," said Laura Correnti, group account director at The Grid (a partnership between OMD and Giant Spoon). Correnti said Serial's audience rivaled the monthly uniques of some of the biggest digital players, such as The New York Times, Vice and ESPN: "Reach of that size is too impressive to ignore."

It's also lifted the audiences for other podcasts. McCrery said most of the top podcasts on iTunes saw a 30 percent audience increase over the last year. "Most of them went looking to say 'what else is out there?'" said Lex Friedman, evp of sales and development at Midroll, which handles advertising for podcasts including Marc Maron's WTF, Scott Aukerman's Comedy Bang! Bang! and Bill Simmons' post-ESPN podcast.

This quarter, Midroll has run campaigns for Chipotle and GE, which is among the biggest to delve into the space, even creating its own science-fiction podcast with Slate’s Panoply network called The Message. "It used to be that we'd go to advertisers and step one was explaining what the heck a podcast is. That happens much less often [now]," Friedman said.

For advertisers, podcasts offer a more intimate space, as hosts have a built-in trust with listeners. A recall study for a Chipotle campaign on Maron's WTF found that 90 percent of listeners could remember those ads.

And there's room for growth. According to Westwood One's "State of American Podcasting" report, 41 percent of advertisers surveyed discussed podcast advertising, but only 15 percent were currently advertising in the medium.

"Because it's in its infancy, I think the ad model is still being worked out," said Correnti.

This story first appeared in the Nov. 30 issue of Adweek magazine. Click here to subscribe.