On July 28, Marc Maron warned listeners of his WTF podcast that he was about to do something new: read an ad for a video game. “Why have I not done an ad for a video game? Because I don’t have any idea what they are!” he said. “I don’t play them. I don’t think I’ve played a video game since, uh, since, like, uh, Asteroids, and Defender, and, uh, Galaga. I don’t think I have. It’s just not my life, all right? It’s just not my life. But these guys want me to do the ad anyways.”

The pitch was for World of Tanks, a free multiplayer online game in which players command tanks on historic battlefields, and WTF agreed to do it as long as Maron could be completely honest. The episode, which included an interview with comedian Mike Myers, went on to become WTF’s third-most downloaded ever. For World of Tanks, it was a smashing success. “Whether Marc was a gamer or not was not really an issue,” says Erik Whiteford, director of marketing for Wargaming America, the company behind World of Tanks. “Actually, we feel that Marc being fully transparent about not being a gamer added credibility to the integrated messaging.”

“They definitely got their money’s worth,” WTF producer Brendan McDonald says.

It’s a funny truth that in the world of podcasting, the ads that get clients “their money’s worth” are nothing like the polished spots that appear on television. They can be rambling and bumbling and offbeat. Words get mispronounced—“MailKimp,” as one person reads in a now-viral podcast ad on Serial, not “MailChimp”—and awkward moments go uncut. The ads are obligatory asides but, when they’re done well, they can also be entertaining mini-segments. Listeners tolerate and even like them. Because instead of companies and their pitchmen appealing directly to potential buyers, most podcast ads have a friendly intermediary: the show’s host. That’s why a growing class of advertisers loves them, too.

“When you listen to a podcast regularly, you really get not only invested in the show, but also in the host,” says Adam Sachs, chief executive at Midroll, a podcast advertising network whose 175 clients include WTF, Comedy Bang Bang, and StarTalk.* “They’re in your ear every day for weeks, and you start to develop a really intimate relationship with the host. And because all our ads are host-read, they work.”

Host-read podcast ads and sponsorships have their roots in the endorsements and sponsor underwriting long used by radio. In the early 1980s, Snapple sponsored Howard Stern’s and Rush Limbaugh’s shows in exchange for on-air endorsements. According to a case study from Harvard Business School, Stern “got to know the founders of the business personally, and conveyed to his listeners a genuine and infectious regard for the products and the people behind them.” John Deighton, the Harvard Business School professor who put together the case, says that at the time, Stern was “really out there on the edge, he didn’t have a national show” but had a small, ravenous listenership. “Rush Limbaugh and Howard Stern were the characters, but they spoke very enthusiastically and ad lib about this product,” Deighton says. “When we choose to listen to someone and then that someone tells us that they like this product, it’s almost word-of-mouth from a trusted friend.”

Thanks to Serial, the podcast spinoff of This American Life that’s become an unprecedented hit (to the tune of more than 5 million streams and downloads from Apple’s iTunes store), the decade-old podcast medium has never been more mainstream. Planet Money. Radiolab. 99% Invisible. StartUp, the first show in a new podcasting venture from This American Life veteran and Planet Money co-founder Alex Blumberg. Earlier this year, Edison Research said in a much-cited report that 15 percent of people ages 12 and over have listened to a podcast in the last month, and that one in five regular podcast users consumes six or more a week. As Kevin Roose wrote in New York magazine in October, “There are too many great podcasts to keep up with.”

While podcasts are exploding, with media organizations (including Slate) adding more shows and new podcast-focused startups launching, the advertisers supporting them still seem relatively few in number. How many different shows have you heard that are sponsored by Audible, or Squarespace, or Stamps.com, or MailChimp?

To some extent, the podcasting medium has placed natural constraints on its advertising opportunities. “There is limited inventory,” says StartUp’s Blumberg. “You can’t just add pages. The more you add minutes, the more you degrade the experience for the user.” Podcasts also tend to attract what’s known as “direct-response” advertisers—companies that use unique URLs or promo codes to track the immediate success of their campaigns—but less interest from ones that use harder-to-quantify, brand-based marketing. And though they’ve been around for a decade, podcasts can still seem exotic to advertisers.  “Even six months ago, maybe a third of my conversations had to start with what a podcast is, and why you should care,” says Lex Friedman, executive vice president of sales and development at Midroll.

As with any form of advertising, it’s also an open question whether the money spent on podcast ads actually works. “It doesn’t matter whether it’s radio, or television, or print—the metrics are just terrible,” says Rohit Deshpande, a professor of marketing at Harvard Business School. “It’s true for all advertising, and it’s true for podcasts.”

At the moment, podcast producers and advertising firms say ads on popular shows are going for somewhere between $15 and $45 CPM (cost per mille, or the price of advertising per thousand listens). The Wall Street Journal reported in November that MailChimp was paying between $25 and $40 CPM for its spots on Serial. CPMs for preroll ads on YouTube, by contrast, average closer to $17. Reply All and StartUp, two offerings from Blumberg’s new venture, Gimlet Media, are pricing even higher—$60 to $100 CPM, says Friedman, who represents the shows.* (Blumberg tells me he prefers not to comment on Gimlet’s CPMs, but says they are “better than we projected they’d be at this point in the company’s development.”)

Friedman thinks the CPMs are high on podcast ads in part because shows have begun doing more customized production on their ad spots, and building that effort into the price. For a recent StartUp ad for MailChimp, for example, Blumberg had the company’s manager of brand marketing give listeners a tour of the MailChimp office. Slate’s Mike Pesca, another guru of the custom ad, has done stunts like promoting Harry’s razors on his show The Gist while shaving with one. Listen and try not to laugh:

Those high CPMs are good news for the podcast industry, which is still working out how exactly it can be profitable. In addition to underwriting shows with ads and sponsorships, podcasts have solicited donations from listeners (as Serial recently started doing) and crowdfunded their way to success (as Planet Money did with a T-shirt Kickstarter campaign that made $590,807).

At a time when consumers are increasingly cynical about advertising, the idea that podcast listeners could laugh at and enjoy podcast ads should be music to advertisers’ marketing-attuned ears. Unlike other forms of advertising, which advertising experts agree we’ve been trained to treat as background noise, we tend not to tune out podcasts. You may flip TV channels, hop between preset radio stations, and click away from browser tabs until a video ad runs through, but how often have you fast-forwarded past a podcast ad? For now, a small but widening group of advertisers has noticed that you’re not doing it too much.

All of the podcast hosts, producers, and advertising executives I spoke to are confident that the interest in podcast ads is only growing. Sachs, the CEO of Midroll, told me his company had 101 active advertisers in 2013 and is now up to 259. The new additions aren’t just direct-response companies, but also include networks like HBO (that pay for “tune-in” campaigns), gaming companies (Cards Against Humanity, for example), and activist groups. Big brands are starting to show up as well. For a while, Planet Money had Ally Bank. WNYC’s Freakonomics Radio is sponsored by Goldman Sachs. Sponsors on Slate podcasts have included Delta Airlines, Acura, and GE. “This is a unique and slightly old-fashioned form of advertising, but also uniquely effective,” says Andy Bowers, the executive producer of podcasts at Slate.

Friedman thinks the commitment of early, empirically minded podcast sponsors has made a case for that effectiveness. “Direct-response advertisers do not spend money foolishly,” he says. “They’re very, very careful, and very numbers-driven.” Translation: If they’re coming back, something must be working. Then again, doesn’t that mean we should be witnessing a flood, not a trickle, of new podcasting advertisers?

“There’s sort of a case where we’ve suggested to advertisers, ‘Why don’t we do a case study to show how effective our ads are,’ ” Sachs says, “and our advertisers say, ‘What, hell no, why would I want to invite other advertisers to come in and compete with us?’ So there’s a little bit of a sense of it being the best kept secret.”

With all due respect to MailKimp, maybe it’s time that secret got out.

*Correction, Dec. 15, 2014: This article originally misstated the number of podcasts Midroll represents. It is 175, not 120.

*Update, Dec. 15, 2014: This article has been updated to include more detailed information on the CPMs of Reply All and StartUp, two podcasts represented by Midroll.