Lots of good info from all the other replies. I just wanted to add my experiences. Usually I've found that if I hit a deer in the heart/lung area, it will drop in it's tracks or only stumble a few yards. Neck or spine shots also (obviously) will drop them. Some exceptions are when a deer is being pushed by other hunters and its adrenaline is flowing. Sometimes then even with a perfect shot, the deer will still run a ways. And obviously, if you don't put the bullet where it needs to be, it's not going to kill the deer right away, no matter what bullet you are using.