Classic is an application in Mac OS X that allows it to run classic applications without having to reboot into the classic Mac OS, and is a descendent of Rhapsody's Blue Box. Classic can load at login, on command, or when a classic Mac OS application needs to be run. It works by loading a full version of Mac OS 9 in a sandbox environment, thus applications that require direct access to hardware may not run correctly.

The name Classic also refers to the application programming environment within the Classic application, and is essentially the same as programming for Mac OS 9. It is one of the five environments available in Mac OS X; the others are Carbon, Cocoa, BSD, and Java.