You can see the What's New in iOS 8 doc for a general overview of, er, what's new: <a href="https://developer.apple.com/library/prerelease/ios/releasenotes/General/WhatsNewIniOS/Articles/iOS8.html#//apple_ref/doc/uid/TP40014205-SW1" rel="nofollow">https://developer.apple.com/library/prerelease/ios/releaseno...</a><p>More specifically, there's the iOS 7.1 to 8.0 API Diffs: <a href="https://developer.apple.com/library/prerelease/ios/releasenotes/General/iOS80APIDiffs/index.html" rel="nofollow">https://developer.apple.com/library/prerelease/ios/releaseno...</a>. Note that a huge number of the changes are just small changes to argument lists (there are a gazillion of these in Accelerate, filling the top chunk of the diff doc) and a number of other change come from the switch from (id) return types to (instancetype) in init: methods. Also, getter and setter methods have been replaced with properties in many classes, which doesn't break any existing code, but generates changes in the API diffs.