Starting today, Apple users that own Macs with macOS High Sierra 10.13.4 shall start seeing a new alert box when a 32-bit app is opened to inform them of coming 64-bit transition.

With macOS High Sierra 10.13.4, the first time you launch an app that does not support 64-bit yields an alert box informing you that the app has not been fully optimized for your Mac.

The alert pops up once per app.

Clicking OK dismisses the warning message so you can continue using the app without restriction. The sole purpose of this dialog box is to give users advance notice that they are running 32-bit software which will not be compatible with macOS in the future.

“New apps submitted to Mac App Store must support 64-bit. Starting June 2018, app updates and existing apps must support 64-bit,” reads a notice on Apple’s website.

The company posted a support document detailing the macOS 64-bit transition. All future Mac software will eventually need to be 64-bit in part because some of the technologies that define the Mac experience, such as Metal graphics acceleration, work only with 64-bit apps.

“You may continue to use 32-bit apps with your Mac today,” reads the document. “Using 32-bit software has no adverse effects on your data or your computer.”

TUTORIAL: How to identify 64-bit apps on your Mac

The iPhone maker underscores that this coming 64-bit macOS transition is still underway so final transition dates have not yet been established.

From the document:

Apple began the transition to 64-bit hardware and software technology for Mac over a decade ago and is working with developers to transition their apps to 64-bit. At our Worldwide Developers Conference in 2017, Apple informed developers that macOS High Sierra would be the last version of macOS to run 32-bit apps without compromise.

While developers optimize their apps for 64-bit compatibility, Apple is notifying customers when they are using an app based on 32-bit technology. This is done via a one-time alert that appears when you launch a 32-bit app.

64-bit apps can access dramatically more memory and enable faster system performance.

Developers who sell apps outside Mac App Store are recommend to distribute 64-bit binaries in order to make sure customers can continue to run their apps on future versions of macOS.

macOS High Sierra 10.13.4 also includes 64-bit testing mode for developers.

iOS 10.3 included a similar message informing users of a pending shift away from 32-bit apps in favor of 64-bit apps. With iOS 11, Apple stopped supporting 32-bit mobile apps altogether.