Dogs bark for all kinds of reasons. They may be alerting you to something noteworthy; making a request (or a demand, depending on your point of view); aggressively warding off (or confronting) prey, predator, or interloper; communicating stress or discomfort; or engaging in play.
Barking might be a response to a howl heard from half a mile away. Or because a squirrel just chattered in the backyard. Or because your dog wants dinner. Or because you walked in the door and life is just! so! good! In any case, barking is a perfectly normal canine behavior.
Behaviorists categorize barking into these categories:
• territorial barking
• alarm barking
• attention-seeking barking
• greeting barking
• socially facilitated barking
• frustration-induced barking
• illness or injury barking
• separation-anxiety barking
But some dogs bark excessively, seemingly at the drop of a pin. Luckily, there are measures you can take to help reduce his or her sensitivity and/or the volume and frequency of the response. Talk to your veterinarian, a board-certified veterinary behaviorist, a certified applied animal behaviorist, or a qualified certified professional dog trainer if you're serious about stopping the noise.
If you believe your dog is barking at absolutely nothing, compulsive barking is a possibility, especially if your dog barks excessively and repetitively while also moving in repetitive ways. For example, a compulsive barker might pace in a circle or walk back and forth along a fence as she barks.
Thankfully, there's help for this problem, too. Ask your veterinarian.