The Alt key on a IBM PC keyboard is either of two keys located next to the Space bar, used to change the function of other pressed keys. Thus, the Alt key is a modifier key, used in a similar fashion to the Shift key. For example, simply pressing "A" will type the letter A, but if you hold down either alt key while pressing A, the computer will perform an "Alt-A" function, which varies from program to program.

The Alt key has come to replace the Meta key of the old MIT keyboards. In their original function, both Alt and Meta would set the high bit of the signal generated by the key to 1 (for example, A generates 01000001 while Alt-A generates 11000001). However, in modern software, due to the requirement of the high bit for internationalisation, Alt no longer works in such a way.

The Alt key is well known as part of the Control-Alt-Delete key combination, which in MS-DOS and Linux is used for restarting the computer, and in Microsoft Windows NT, Windows 9x and derivatives, brings up the task manager for aborting an irresponsive application.

Some programs treat both Alt keys on the keyboard as the same key, while others do not.