The Supreme Court has agreed to hear Apple’s appeal in an antitrust lawsuit over the App Store’s structure and fees. Reuters reported the news this morning, and it marks a major step for the case, which has been running since 2011. Bloomberg writes that the court will hear arguments within a nine-month window that starts in October.

The case, Apple v. Pepper, argues that Apple’s App Store monopolizes the distribution of iOS apps since the store is the only officially approved way to download an iPhone or iPad app, and Apple charges developers a significant 30 percent fee. Its plaintiffs are seeking class action status for the lawsuit, saying that this fee has driven up the costs of apps for consumers. A lower court determined that they had no grounds to sue, but a judge overturned that decision, and the Supreme Court began seeking advice on the case late last year. Now, the court is officially taking it up.

Apple has argued that it created a new competitive market, and it’s said before that its phones’ ecosystems are closed for security reasons since it can vet App Store apps for malicious code and other dangers. The Department of Justice has filed a supporting brief backing up the company. If it loses the case, Bloomberg cites a lawyer claiming that Apple might have to pay hundreds of millions of dollars. And beyond that, the suit is taking aim at the whole concept of app store walled gardens, so other companies with similar marketplaces may also be watching for a verdict.