Are Cavalier King Charles Spaniels good with cats?

Yes, Cavalier King Charles Spaniels are known to be good with cats. Their friendly and adaptable nature typically makes them open to forming positive relationships with other animals, including cats. 

Of course, successful cat-dog interactions often depend on the individual personalities and temperaments of both the dog and the cat. But you can feel fairly confident that if properly socialized to your cat and/or introduced at a young age, your Cavalier King Charles Spaniel should get along just swell.