I brought an 8 year old Border Collie/Retriever mix in w/my two cats who were 9 and 10 at the time. Mickey (the dog) could have cared less. The cats did spend a little time higher up on the cat condo for a bit until they figured out the dog wasn't interested in them. Now my dog had been around cats before she also does try to chase other cats if she is outside but inside the house, they're just a piece of moving furniture to her.