Tail biting is a behavioural problem of pigs which is associated with welfare detriment for both the perpetrator and victim. It is seen to some extent on most farms worldwide and causes considerable economic loss, leading to widespread adoption of tail docking for risk reduction. Its occurrence is often sporadic and unpredictable, as a result of the many different combinations of chronic and acute risk factors which can be present on individual farms. Understanding of the underlying (neuro)physiological mechanisms which lead an individual pig to initiate tail biting is still incomplete, but stress, pro-inflammatory cytokine production, changes in amino acid metabolism and serotonergic brain pathways have been implicated. Rearing pigs with undocked tails and without tail biting is still challenging in commercial practice and requires a high quality of management and stockmanship to minimise risk, detect early warning signs of biting and intervene appropriately.