From the link:
QUOTE
iPhone (models 4S, 5, 5S, or 5C), iPad (models 2, 3, 4, including the Air, and all versions of the Mini), or iPod Touch (5th generation only)
Of course, that doesn't mean that an app that needs newer
hardware will run, even with updated
software.
That's why you should always have a
Restorable backup.
Edited (9/16/2014) to include a new comment at the linked site:
QUOTE
...While the devices will run the OS, you may not see things go quite as snappy as expected, and perhaps be slower than your device running iOS 7 right now. Therefore, if you have an iPad 2, or an iPhone 4S, consider waiting to hear other accounts of how the new OS performs...
Edited (9/17/2014) to include info at the linked site:
QUOTE
download ~1.1GB, installed space ~5.8GB!
If you are close to maxing out your available RAM, you might as well start deciding what to remove!
Make sure your battery is charged.
Even the 'install' process took nearly 1.5 hours! Don't plan on doing the upgrade during business hours or when you might need the phone!
Humorous report at: <
What to expect when upgrading to iOS 8> "Forewarned is four armed" or something like that...