The apps that you've purchased from the iTunes App Store -- along with all the music, movies and television shows you've bought from iTunes -- are tied to your Apple ID. If you haven't authorized your computer, it won't be able to sync or use content you've purchased from the iTunes Store. To prove that you're authorized to download the items you've purchased, you must sign in and verify your identity on each of the computers you want to use iTunes on.
You can authorize a computer from within iTunes. Open the "Store" menu and select the "Authorize This Computer" option. You're prompted to enter your Apple ID email address and password to verify that you're entitled to download and use the apps that you've purchased. If iTunes is already associated with a different Apple ID, you'll lose the ability to use the content linked to that account. Only one Apple ID account can be supported at one time.
Managing Authorized Computers
You can authorize up to five different computers with one Apple ID; these computers can be a mixture of Windows and Mac OS machines if required. You can deauthorize a computer to free up a slot using the option on the tore" menu in iTunes. To deauthorize all of the computers linked to your Apple ID and start afresh, open the iTunes Store within iTunes and then click the "Account" link. You'll be prompted for your Apple ID email address and password, and you can then access the Deauthorize All option from the account page.
Apps on iOS Devices
An Apple ID is required when setting up any iOS device, enabling you to download the apps, music, movies and other content that you've previously purchased. To check which Apple ID your device is associated with, and change it if necessary, open the Settings app and tap the "iTunes & App Store" link. The current Apple ID is shown at the top of the screen; tap it to sign out or to view more details about your account.
If you experience problems when trying to authorize your Apple ID with iTunes, use the "Check for Updates" option on the Help menu to verify you're running the latest iTunes version. You can also try authorizing iTunes from an administrator account or with an administrator password on your computer, or turning off "User Account Control" if you're running Windows Vista or Windows 7. Reinstalling iTunes may also be enough to resolve the issue. To check which Apple ID an app is associated with, right-click on it and choose "Get Info."
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