How hard is it to take a dog from US to Canada?

Just curious. My parents are visiting my cousins in CA, and they are trying to find a home for a dog. Her name is Noel, and she belonged to a friend of theirs who passed away recently. She's 5, and looks really sweet. I don't have much information, and I'm sure this won't work out anyway (my parents always fall in love with some kind of stray that my cousins have when they are down there!).

But it made me think, is it that hard to bring a dog across the border?