The streaming music wars continue to benefit music fans. In the latest salvo, Rdio announced today that United States users can listen to all 20 million songs on the service for free on the Web.

But listeners will have to put up with advertisements, according to a Rdio blog post:

As part of this update, we’ve added in-stream messaging to Rdio on the web. These new ads are short and sweet. Free listeners will hear a mix of new feature announcements, messages from partner brands, notifications about exclusive content, and other helpful tips.

That’s great news for music fans. On the iOS side, you’ll have to download the universal app designed for the iPhone/iPod touch and iPad/iPad mini. While Pandora-like radio stations are free, you’ll have to become a Rdio Unlimited subscriber to listen to other songs.

When you pay the $9.99 per month subscription, there will be no ads on either the Web or app.

Rdio’s announcement is thought to be in response to Spotify, which has been especially aggressive adding free music. Earlier this month, the competing service updated its iOS app to add a free music service for listeners.

After Apple officially introduced iTunes Radio in 2013, the landscape has become much more competitive for both free and paid music services. And things will become even more crowded later this month when Dr. Dre’s Beats Music is set to launch.