Force Quit

To force quit an application program is to quit the program without saving changes to any unsaved documents or settings. This is an emergency tactic and a troubleshooting technique on the Macintosh.

Keystroke
In macOS and Mac OS X, the keystroke to force quit any application program (including, to an extent, the Macintosh Finder) is command-option-escape.

Usage
When an application program quits normally, the user is prompted to save any documents that have newly inputted or modified changes which have not yet been saved. The user can then choose to save or not to save the changes, and a large majority of programs also offer an option to cancel the quitting process.

When an application is frozen or stops behaving normally, the user may not be able to quit the application normally. As a result, in order to force the program to quit, the user is left with the sole option to force the application to quit. This would not allow unsaved changes to be kept; such changes are only stored in memory, to be gone once the application is force quit. This technique is also used to escape malware pages without any harm.

Classic Mac OS
Because classic Mac OS is based on cooperative multitasking instead of pre-emptive multitasking, it was common for a system error in one application to freeze the entire system, forcing the user to restart the computer. However, it is possible to invoke a debugger through  (or a programmer's key) to enter commands to force quit the crashed foreground application that caused the error condition.

In early systems with 24-bit addressing, the following can be typed into the debugger:

In 32-bit clean or classic PowerPC-based systems, the recommended memory location was modified:

As of Mac OS 8, once an application has been forced to quit, a dialog box will warn the user that the system may be unstable, and advises to save documents and restart the computer.

Mac OS X
Mac OS X is a fully pre-emptive multitasking operating system. Force quitting applications in Mac OS X, therefore, cannot destabilize the entire system. Instead, only the application that has become frozen is affected without locking up other applications. An application can be force-quit by using the Force Quit... command in the Apple menu or by option-clicking the application icon in the dock.

As of Mac OS X 10.3, adding the shift key to the command-option-escape keystroke forces the current application to quit.

In Mac OS X 10.4 Tiger, even Dashboard widgets are given equal treatment. It is also possible to determine unresponsive applications (or invisible processes) from Activity Monitor and force quit them from there.

Forced Force Quitting
Under the following circumstances, a user may be obliged to force an application to quit, even when the program is not acting up or is being unresponsive:

Classic Mac OS as of Mac OS 8: A red dialog box may appear, which alerts the user that there is no more free memory available. If the user clicks Restart, all applications are potentially forced to quit, and the system is restarted nearly instantaneously.

Mac OS X: Under very rare circumstances, virtual memory can eat away all memory, leaving no more free space on the hard drive. The only way to continue working is to quit an application.