Tail docking involves amputating a portion of the tail for a variety of reasons. We review the scientific evidence for the rationale for tail docking, a description of the different methods used, the pain response to the procedure and the effectiveness of pain alleviation, and, finally, the alternatives to tail docking and policy regarding the practice. We focus on the three main agricultural species that are tail docked as a management practice: pigs, sheep, and dairy cattle. Methods of tail docking include cutting with a knife or scalpel, cutting with a hot docking iron, or application of a constrictive rubber ring. All methods are commonly performed without analgesia or anaesthesia, and all likely result in some degree of pain. As with any procedure that alters the integrity of an animal, it is important to consider the rationale behind docking in order to evaluate if it is necessary. Tail docking in pigs is routinely conducted on commercial swine farms because it can reduce the incidence of tail biting, an injurious and undesirable behaviour. Both behavioural and physiological changes indicate that tail docking is painful in pigs, but until robust and consistent methods for preventing tail biting are identified, this procedure is likely to continue as a management practice. This approach is reflected in public policy about the procedure. There is both behavioural and physiological evidence that tail docking is painful for sheep; both responses are reduced when pain relief is provided. Prevention of fly strike is the primary reason given for tail docking sheep, but the scientific evidence to support this rationale is surprisingly sparse. Further research is required to justify tail docking of sheep as a routine practice. Dairy cattle are docked because this practice is thought to improve cow cleanliness and udder health, however, there is no scientific evidence supporting this rationale. Tail docking cattle results in relatively few behavioural or physiological indicators of pain, but docked cows are unable to effectively remove flies from their hind end. The practice of tail docking dairy cattle is banned, discouraged or declining in most industrialized countries except the US. The long-term pain associated with tail docking is not well understood in pigs, sheep or cattle. In cases where tail docking may be justified by demonstrated benefits for the animal (possibly in case of pigs and sheep), further research is needed to find either practical alternatives or ways to alleviate the pain associated with this procedure.