
Any of the following warnings mean the dog is in an aggressive state of mind.

Photo by Matthew Roberts
• Curling lips, showing teeth,
• The tail held up high, and (sometimes) wagging,
• The head carried up high with the ears held straight up,
• Body positions that make the dog look taller and bigger,
• A very alert and still body posture with the back legs held further out from the body is a sign that the dog is upset and may become aggressive,
• Lunging,
• Snapping,
• Blocking your path,
• Unfriendly barking,
• Biting even if it does not break the skin.
If your dog occasionally shows one or some of these signs, or if it has been one-time occurrence, you can most probably deal with the problem yourself by setting up rules and boundaries, obedience training, socializing your dog and using positive reinforcement techniques. Before you start acting, it's important to determine whether your dog is showing dominant, territorial, fear, or food (prey, predatory) aggression. It should help you choose the best way of dealing with the problem.
If you don't think you can deal with your dog's aggression yourself, or if you become seriously worried about your own and your family's safety, it's highly recommended that you get professional help.
See also: Children Think Angry Dogs Are Smiling