If you've encountered a problem with macOS, the solution might be to reinstall the system on your Mac. This can be done by any Mac owner, but it can be a little bit complicated. You'll need to put your Mac into macOS Recovery, which gives you tools to diagnose problems on your Mac, as well as reinstall the operating system.

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Here's how to reinstall macOS using Recovery mode.

Entering macOS Recovery

macOS Recovery has tools at its disposal that allow you to diagnose problems with your Mac's hard drive, reinstall macOS, and restore from a Time Machine backup. You can enter Recovery by pressing certain key combinations. The combination you choose will determine which version of macOS will be installed on your system when you select Reinstall macOS.

• Press Command - R to install the latest version of macOS that was installed on your Mac without installing a later version.
• Press Options - Command - R to upgrade to the latest version of macOS that's compatible with your Mac.
• Press Shift - Option - Command - R to install the version of macOS that came with your Mac, or the version closest to it that's still available.

How to check your startup disk with Disk Utility in macOS Recovery mode

The first thing you'll want to do is make sure your Mac's startup disk is healthy. You'll need to use Disk Utility to do that.

1. Click Disk Utility when in macOS Recovery.
2. Click Continue.
3. Click on your Mac's startup drive in the Disk Utility sidebar. It should be the drive at the top of the sidebar if there are multiple drives.
4. Click First Aid.
5. Click Run. First Aid will run, checking the health of your Mac's drive. This process could take a little while.
6. Click Done.
7. Click Disk Utility in the Menu bar.
8. Click Quit Disk Utility.

How to erase your startup disk in macOS Recovery mode

If you need to start completely fresh, then you'll want to erase your startup disk using Disk Utility. If you want the content on your Mac to stay intact, skip to the next section.

1. Click on your startup disk in the Disk Utility sidebar.
2. Click Erase.
3. Click the Format drop-down menu.
4. Click on the format you want to use for your drive. If you're running macOS High Sierra or later on a Mac with an SSD, your choices will be some form of APFS.
5. Click Erase. Your drive will proceed to be erased.

How to reinstall macOS while in Recovery mode

The next step is to reinstall macOS.

1. Click Reinstall macOS.
2. Click Continue.
3. Click Continue.
4. Click Agree.
5. Click Agree.
6. Click on the drive on which you wish to install macOS.
7. Enter your Apple ID and password if you're asked to, though you might not be.
8. Click Install.

Your Mac will now go through the normal macOS installation process, eventually restarting. If you're starting from scratch, you'll need to set up your Mac from scratch.

Questions?

If you have any more questions about reinstalling macOS on your Mac, let us know in the comments.

Updated December 2019: Everything is up to date through macOS Catalina.

macOS Catalina

Main

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To use any of these key combinations, press and hold the keys immediately after pressing the power button to turn on your Mac, or after your Mac begins to restart. Keep holding until the described behavior occurs.

• Command (⌘)-R: Start up from the built-in macOS Recovery system. Or use Option-Command-R or Shift-Option-Command-R to start up from macOS Recovery over the Internet. macOS Recovery installs different versions of macOS, depending on the key combination you use while starting up. If your Mac is using a firmware password, you're prompted to enter the password.
  
• Option (⌥) or Alt: Start up to Startup Manager, which allows you to choose other available startup disks or volumes. If your Mac is using a firmware password, you're prompted to enter the password.
  
• Option-Command-P-R:Reset NVRAM or PRAM. If your Mac is using a firmware password, it ignores this key combination or starts up from macOS Recovery.
• Shift (⇧): Start up in safe mode. Disabled when using a firmware password.
• D: Start up to the Apple Diagnostics utility. Or use Option-Dto start up to this utility over the Internet. Disabled when using a firmware password.
• N: Start up from a NetBoot server, if your Mac supports network startup volumes. To use the default boot image on the server, hold down Option-N instead. Disabled when using a firmware password.
• Command-S: Start up in single-user mode. Disabled in macOS Mojave or later, or when using a firmware password.
• T: Start up in target disk mode. Disabled when using a firmware password.
• Command-V: Start up in verbose mode. Disabled when using a firmware password.
• Eject (⏏) or F12 or mouse button or trackpad button: Eject removable media, such as an optical disc. Disabled when using a firmware password.

If a key combination doesn't work

If a key combination doesn't work at startup, one of these these solutions might help:

• Be sure to press and hold all keys in the combination together, not one at a time.
• Shut down your Mac. Then press the power button to turn on your Mac. Then press and hold the keys as your Mac starts up.
• Wait a few seconds before pressing the keys, to give your Mac more time to recognize the keyboard as it starts up. Some keyboards have a light that flashes briefly at startup, indicating that the keyboard is recognized and ready for use.
• If you're using a wireless keyboard, plug it into your Mac, if possible. Or use your built-in keyboard or a wired keyboard. If you're using a keyboard made for a PC, such as a keyboard with a Windows logo, try a keyboard made for Mac.
• If you're using Boot Camp to start up from Microsoft Windows, set Startup Disk preferences to start up from macOS instead. Then shut down or restart and try again.

Remember that some key combinations are disabled when your Mac is using a firmware password.

Learn more

Microsoft Activation Support

• Learn what to do if your Mac doesn't turn on.
• Learn about Mac keyboard shortcuts that you can use after your Mac has started up.