The podcast industry earned more than $300 million in ad revenue in 2017, an estimated $400 million in 2018, and is expected to exceed $600 million by 2020, according to the Interactive Advertising Bureau.

That’s small compared to newspaper and radio ad sales — about $34 billion total, but big considering podcast advertising was almost nothing a few years ago.

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You can’t talk about the podcast ecosystem without starting with Apple, Glenn Fleishman wrote for Fast Company.

The Apple Podcasts app arrives in your hand as part of iOS, the operating system used for mobile devices manufactured by Apple Inc. That makes it available to a billion-plus iPhone and iPad users. Listeners who use this app represent 70-percent plus of podcast consumers. All other apps have just a few percentage points of listeners, according to Fleishman.

There isn’t a way for podcasters to charge for content, but podcasters can get on the Apple directory free and listeners get a streamlined way to subscribe to new episodes and choose from back catalogs.

“I get it. Apple’s default podcast app is easy. It’s preinstalled, it’s free, and it gets the job done. But if you stick with what’s easy, you’re not only letting Dumbledore down, you’re missing out on some amazing apps,” Abu Zafar Wrote on Life Hacker. Zafar tested eight of the most popular podcast apps on iOS, including Apple Podcasts, and ranked each on three criteria: their visual aesthetic, the features they offer, and how easy it is to discover new podcasts within the app. You can see the results here.

When it comes to podcast analytics, it’s a free-for-all. “There’s no massive, third-party, arbiter of success, like Nielsen for TV ratings, that gives advertisers a clear idea of how a show measures up to its competition,” Ashley Carman wrote for The Verge. “Right now, advertisers rely almost entirely on download figures to determine the value of advertising on a podcast.”

Counting advertising alone misses an important and growing part of podcast revenue, according to Fleishman. Podcasters earn other forms of income for paid content that’s included as part of a subscription or charged as a premium. This revenue is collected by podcast apps on iOS, including Audible, Spotify, and Stitcher.

Podcast apps with specialized features have proliferated. Some target listeners, and others are trying to reach show producers and podcast networks. They’re trying to accept payments, reward listener loyalty, and gather metrics outside of Apple’s constraints.

The apps don’t give details about the income they earn selling podcast content, Fleishman said. It could reach billions of dollars in a few years, based on rapidly growing demand.

The potential listening market is huge. In 2006, 11% of Americans age 12 or older had listened to a episode. By 2018, that number had reached 44 percent, — 124 million people, according to Edison Research.

Advertisers will expect more detailed listener data as the industry matures, said Dennis Buchheim, who leads the Interactive Advertising Bureau (IAB) Tech Lab. “As the podcast industry scales, advertisers will look for measurement and attribution techniques available in other forms of digital media.”