As we previously wrote about, 32-bit Mac apps are being phased out by Apple. Well, that transition officially began today. As of now, anyone running macOS High Sierra 10.13.4 will begin seeing messages alerting them to any 32-bit apps they’re using.

This alert appears once per app only.

The first time an application is opened while on 10.13.4, a message reading, “This app needs to be updated by its developer to improve compatibility.” appears. It can be dismissed by clicking “OK”, and will not reappear the next time the application is opened. The popup serves only to inform the user that the application in question has not been optimized for 64-bit use, and in future versions of macOS, it will not be compatible.

Per Apple’s announcement, “new apps submitted to the Mac App Store must support 64-bit, and starting June 2018, app updates and existing apps must support 64-bit.” The reason driving the transition are many-fold, but in part, many of the technologies that make up the user experience, including things like Metal graphics acceleration, are driven now by 64-bit architecture.

You can continue to use any 32-bit apps installed on a system for the time being, and they won’t cause any adverse effects. But at some point in a future that’s rapidly approaching, those programs will either need to be updated or their users will need to start looking for a replacement.

A similar iOS transition has already taken place. When iOS 10.3 was released, a similar message greeted mobile users operating any apps with a 32-bit architecture. As of iOS 11, those apps are no longer supported.