OS X Mountain Lion: Erase and reinstall OS X
You can erase your Mac and then reinstall OS X using the built-in recovery disk. Before you erase and reinstall, back up your essential files. If you’re installing on a portable computer, make sure your power adapter is connected and plugged in.
- Choose Apple menu > Restart. Once your Mac restarts (and the gray screen appears), hold down the Command (⌘) and R keys.
- If you’re not connected to the Internet, choose a network from the Wi-Fi menu (in the top-right corner of the screen).
- Select Disk Utility, and then click Continue.
- Select your startup disk from the list on the left, and then click the Erase tab.
- From the Format pop-up menu, choose Mac OS Extended (Journaled), enter a name for your disk, and then click Erase.
- After the disk is erased, choose Disk Utility > Quit Disk Utility.
- Select Reinstall OS X, click Continue, and then follow the instructions.
For more information about reinstalling OS X, see this help topic:
You can also create an external recovery disk. For more information, see this Apple Support article: