I've never known an aggressive Collie. My neighbor had one that was protective of his fenced yard, but that's not aggression. He nipped me on the butt once when I was in his yard. Collies are protective, they are guardians. I had a Collie once, too. Great babysitter for my kids when they were small. But gentle, patient, tolerant. I heard once they could not use them in the military bc they could not teach them to attack on command. They will only attack if they perceive a threat. I can't verify that, but I believe it.