Here is my last Rot, a wonderful female
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I am going to echo what everybody else has said. Pick your breeder, look at both parents. Some have been bred for big bodies, giant heads, and aggression. Steer clear. Hip dysplasia is another problem--the parents should have certificates showing them to be free of the problem. Avoid any rescue dogs--they probably weren't well trained. Don't get a Rottie if you don't know how to train dogs. Dominance is an issue--You have to be the alpha. That doesn't mean you beat them to show who is boss, but you need to have the time and know the techniques. Obedience training is an art and a science--if you don't know how, sign up with a trainer who welcomes big dogs.
If you get a good one and train it right, you get a friend and protector without equal.