Docking or bobbing is the removal of portions of an animal's tail. It should not be confused with cropping, the amputation of ears. Tail docking may be performed cutting the tail with surgical scissors (or a scalpel) or constricting the blood supply to the tail with a rubber ligature for a few days until the tail falls off. The length to which tails are docked varies by breed, and is often specified in the breed standard. Docking is illegal, or restricted, in many countries. Some dog breeds have naturally occurring bobtail lines. These appear similar to docked dogs but are a distinct naturally occurring phenotype.

Historically, tail docking was thought to prevent rabies, strengthen the back, increase the animal's speed, and prevent injuries when ratting, fighting, and baiting.

Tail docking is done in modern times either for prophylactic, therapeutic, cosmetic purposes, and/or to prevent injury. For dogs that work in the field, such as some hunting dogs, herding dogs, or terrier dogs, tails can collect burrs and foxtails, causing pain and infection and, due to the tail's wagging, may be subject to abrasion or other injury while moving through dense brush or thickets. Bones in the tail can also be broken by pulling or impact in the field, causing spinal injury to the tail. The American Veterinary Medical Association (the largest veterinary professional organization in the United States), disputes these justifications, saying "These justifications for docking working dogs' tails lack substantial scientific support. In the largest study to date on tail injuries in dogs the incidence was 0.23% and it was calculated that approximately 500 dogs need to be docked to prevent one tail injury."

Docking of puppies at 3 to 5 days old is routinely carried out by both breeders and veterinarians without anesthesia.^[where?] Risks from anaesthesia are highest in a puppy's first 2 weeks of life, and puppies return to normal behaviour on average 3 minutes following the procedure; therefore anaesthetic is not considered a procedural improvement, nor does it mediate any longterm physical or behavioural effects of docking.

While the tails of some working dogs are docked to prevent injury or infection, the tails of larger dogs commonly used for guard work or protection work (not to be confused with patrol work where a handler can provide secondary aid) may be docked to prevent their tails from being grabbed in a fight. This is most common in the Rottweiler, Doberman Pinscher, Bandog, Cane Corso, Boerboel, etc.

Robert Wansborough found in a 1996 paper that docking tails puts dogs at a disadvantage in several ways. First, dogs use their tails to communicate with other dogs (and with people); a dog without a tail might be significantly handicapped in conveying fear, caution, aggression, playfulness, and so on. Leaver and Reimchen, in 2007, found that longer tails were more effective than shorter tails at "conveying different intraspecific cues, such as those provided by tail motion".

It has also been suggested that certain breeds use their tails as rudders when swimming, and possibly for balance when running; so active dogs with docked tails might be at a disadvantage compared to their tailed peers. Videos comparing docked and undocked dogs running and jumping show that dogs who are docked have to work harder to compensate for the loss of the tail. Canine pathologist and sports-medicine expert Prof. Chris Zink believes the extra stress imposed on the joints can have long-term health consequences.

In 2007, Stephen Leaver, a graduate student at the University of Victoria, published a paper on tail docking which found that tail length was important in the transmission of social cues. The study found that dogs with shorter tails (docked tails) would be approached with caution, as if the approaching dog was unsure of the emotional state of the docked dog. The study goes on to suggest that dogs with docked tails may grow up to be more aggressive. The reasoning postulated by Tom Reimchen, UVic Biologist and supervisor of the study, was that dogs who grew up without being able to efficiently transmit social cues would grow up to be more anti-social and thus more aggressive.