Some customers may be unable to make purchases, which is in line with reports about downtime on Twitter. We've also heard complaints about Apple Music and other services, though Apple isn't listing problems outside of the App Stores.
Apple's developer website confirms that TestFlight and App Store Connect are affected as well, so developers and testers may be unable to use these services at the current time.
The outage has been ongoing since 1:40 a.m. Pacific Time, and it's not clear how many customers are affected. We don't know when the issues will be fixed, but we'll update this post when all services are up and running.
Update: According to Apple's System Status page, the issues affecting the App Store, Mac App Store, iTunes, TestFlight and App Store Connect have been resolved.