The podcast industry is growing fast; the industry generated an estimated $479.1 million in revenue in 2018 and is expected to produce more than $1 billion by 2021, according to a new report from the Interactive Advertising Bureau (IAB) and PwC. The latest revenue figure represents an increase of 53 percent in just a year.

The shift to podcast ads backs up what industry titans, like Spotify, have been saying for months. The company plans to spend up to $500 million this year on podcast-related acquisitions. It’s acquired not only content studios — Parcast and Gimlet Media — but also a creation app — Anchor. Another Spotify-owned entity, Soundtrap, also released a new tool specifically designed to help with podcast creation. The Spotify team has already said it plans to build out its podcast ad business, which could end of conquering a decent portion of the revenue.

Multiple startups are trying to get in on all aspects of the podcast industry, too, including the tech, creator, and platform side. Luminary and Himalaya each raised $100 million in venture capital; podcast marketing tech company Chartable raised $1.5 million; a new podcast creators fund, PodFund, raised $2.3 million; and already established companies, like Sony Music Entertainment are entering the space.

Podcasters are slowly transitioning to dynamic ads, which allows them to change ad content over time as opposed to having one ad baked into a show for its entire existence. Dynamically inserted ads now account for 48.8 percent of podcast ads business, the report says.

But with that said, today’s report says that host-read ads remain the most popular form of advertising, and those require specific relationships with the podcasters and their shows. Meanwhile, some big companies are trying to build out their podcast ad business. Spotify’s ad formats, for example, could involve branded content in the form of specially made shows, or inserted ad breaks for non-premium users, as well as host reads from Spotify-exclusive shows. It’s unclear how much interest advertisers will have in these formats versus the traditional host read, but regardless, Spotify will likely have ample space to pitch advertisers to try and generate more revenue.