Dogs pant. It's their way of cooling their body. While humans have sweat glands that produce perspiration to cool the body when it overheats, dogs cool off by panting and sweating through the pads of their paws. Panting plays other important roles, such as bringing in large amounts of air during or after exercise, a way to alleviate stress in fearful situations, compensation for pain, or a show of excitement. Breeds with shorter snouts, such as pugs, bulldogs or pekingese, have more difficulty breathing and may pant more than other breeds.
It is important to distinguish normal panting in your dog versus panting that may indicate a problem. Some dogs just pant more than others but this does not mean there is something wrong. If you are unsure whether or not your dog's panting is a problem, consult with your veterinarian.
Though panting is normal in situations such as heat and exercise, excessive panting is a red flag. If you notice your dog is panting more than normal, take notice of surrounding factors. Reasons for excessive panting include (and should be brought to the attention of a veterinarian immediately):
Other signs to look for if your dog is excessively panting:
the panting started all of a sudden with no reasonable explanation
the tongue or gums are blue/purple or white (this indicates a lack of oxygen)
constant, heavy panting with no break
a wound or injury
All these signs warrant an immediate trip to the vet.
Note: Dogs are like humans when it comes to exercise: they need to build up their endurance. When starting your dog on an exercise regime, begin slowly and work up to long jaunts. Excessive panting could have to do with an out-of-shape dog.