This small breed is generally brave and smart, and though he's a member of the Toy Group, the Yorkie is truly a Terrier at heart. This breed can be playful and affectionate but tends to lay down the law, even with larger dogs in the home, and typically offers a boisterous and determined alarm when someone new comes to visit.
Remember: Every dog is an individual, and his or her behavior comes from a variety of things, including genetics, training and owner experience.
That said, many of these breeds are prone to barking, so consistent, positive reinforcement training should be used to teach these (and all) dogs about appropriate barking.