Q: iTunes will not let me sync apps to my iPhone because it says I am not authorized on this computer, however my account is authorized!?

Well my "solution" did not work very long. I still have the same problem.

This worked for me, a lot faster than redownloading.

ok here is the deal.i tried almost all of the above mentioned solutions but it did not work for me.what actually worked was i deleted the one ''leech'' from my iphone,,the only app which i did'nt download from itunes or app store rather from some hack software.and boom it worked.as soon as i deleted that app from my phone it started syncing again.i think its some new apple policy cz it worked fine with the previous version of itune.i downloaded the latest version of itune and this problem came.

so guys delete all the hack softwares from your idevices and then try again it will definitely work for all of you!!!

Thanks mosherry!!!..I was having the same problem discussed above and tried all the methods..but as soon as I deleted those apps downloaded from other apple ID..syncing started!!!!

My pleasure 

Yes after hours I finally figured out a solution with all this authorization glitches. So basically every one is getting this message from downloading an app or song from a different computer that was not originally synced with there device originally, or have downloaded an app or song not on there iCloud. To solve this , sadly you must delete or back up to the latest backup so that those apps or songs are no longer on your device. Hope this helps everyone, it does suck having to re download or buying an app you already bought but it is nice having no authorization glitches or loop holes. I hope you all think this is helpful.

What solved it for me was to select "Manually manage music and videos" in iTunes/mydevice/summary!

I didn't have to remove any offending app. What it does, is just re-sync everything that's on the computer already, without the annoying "authorize this computer" message.

Please note: I made sure to "transfer purchases" before hand.

Not sure if this is key or not, but as I was trying a bunch of different things suggested in this discussion,

here's what I also did before succeeding:

- making sure my ID was the same on my device & on my computer

- authorizing different IDs on my computer again & again

- turning on "Automatic Downloads" (all 3 of them) in Settings/iTunes & App Stores on my device

Hope this works for some of you too.

Edit to my message above:

Once "Manually manage music and videos" is selected, next time you open iTunes it'll  give you the "not authorized" message again. Simply deselect "Manually manage music and videos", and the sync will go smoothly again.

So I guess the selecting or deselecting must be done each time you want to sync... Not great, but easier than other solutions suggested imo.

Well I've got my iPhone5S after 3 weeks of hunting it down. Meanwhile I was using my other Apple ID - I am using for development and my old iPhone4. By some strange luck I've got my new iPhone5S not in my hometown during my traveling. So, right in the store I backed up my very limited account with 2-3 free Apps to the cloud and restored it back to my new phone. few days later I was home and I connect it to iTunes and pushed Restore From Backup (I have backup of my previous iPhone4S before I sold it). Restore was ok and quick, but I stuck with the same error - topic of this thread. After trying to follow 3-4 proposed solutions I was still sitting in front of the same error window.

My solution: I suspect that some apps/files/settings still remains in my phone from old apple ID. I've ERRASED phone completelly and during setup questions I asked to setup as restore from iTunes backup. It asked me to connect to iTunes and Ha! - I am watching my almost 300 apps are perfectly syncing...

So, here you all have it - simple restore of a backup on top of different apple id iphone will not work. At least for now - Oct 20, 2013. Errase your phone and restore it clean - everything will work.

I HAVE FOUND THE SOLUTION !!

read carefully, ! i had been trying for a long period to backup/sync my apps to iTunes on my laptop from my iPhone 5 iOS 7.  i even authorized computer. but the problem was still here, Keep remember, To sync/backup apps, all apps should have been downloaded using same apple id that u used to authorize the computer,  and i had same id on all my apps on iphone and same id on itunes authorization. so what was causing the problem? i found it,

i realized that i had 3 cracked apps on my iphone, don't wanna mention much. obviously these aps were illegal and were not linked anyway to my apple id. so these were main problem that was not leting my all other apps to sync to iTunes. SO I DELETED THESE 3 CRACKED APPS. Stop doing illegal stuff.
now i am already syncing my all apps, successfulyy, its working fine. Only because iTunes is linked to same apple id, and all apps on iphone are linked to same apple id.
SO its working.

Hi,

I was getting the "Some of the apps in your iTunes library were not installed on the iPhone "my iphone" because one or more errors occurred.  I have recently erased my Sprint iPhone 4s because I switched to Ting.com. 

What I discovered was that some of the apps that are in my iTunes account are no longer on my pc's hard drive.  I found this out by going to the Apps section of the Library, then click on the "List" button, near the top right.  Then I noticed that I have an exclamation inside a circle to the left of a bunch of apps.  I double-clicked the exclamation mark and received a pop-up dialog box with the message "The app "Evernote Hello" could not be used because the original file could not be found.  Would you like to locate it?"  I tried searching for the correct .ipa file but I could not find it on my pc. 

To fix this:

1.I clicked the ">" symbol and then clicked on "Show in iTunes Store".  This brought me to the iTunes page for this app.  The button says that it is already downloaded. 

2. So I clicked on the "Apps" section of the Library, right-clicked the app and deleted the app, switched back to the "iTunes Store" section of the Library, clicked the "Evernote Hello" link at the top of the screen. 

3. This caused the page to refresh, and now the "Download" button shows up and I re-download the app that is not on my hard drive.  .

I am going to do this with 78 other apps, one by one.  I did notice that one of my TWiT apps is no longer available on the iTunes store, so I just deleted that one, don't know if they pulled the app or if it's not available for ios7.

There must be something wrong with my iTunes library on my pc and I'm not sure of a way to have it fix itself.  If someone else knows, please let me know.

p.s. If you decide to try out ting.com as a super low cost cell phone service for people that rarely use their phone, as a cell phone, and basically always use wi-fi, please sign up using my referral code.  Both of us will get a $25 credit.  FYI, they use Sprint as their service provider.  Check it out at https://z9o3b52hlc5.ting.com/

My cell plan is at $15 for the month. :-)

Thanks so much ocarlsen!  This was my problem.  I had downloaded some apps before I changed my ID to match across all devices.  Soon as I followed your directions the 4 giving issues backed right up on iTunes!  Thank you for alleviating months of frustration for me!!

All you need to do is to change the Backup on ur Itunes !
I Changed it from backup to this computer to Backup to Icloud and I no longer recive this annoying message that I need to authorize to this computer.
Hope this helps you as well.