Sport has provided some of the most abiding images of the Great War. The impromptu football played between British and German soldiers during the 1914 Christmas Truce, the British troops kicking footballs across No Man’s Land at Loos and at the Somme, still resonate in the public memory. For the British army the war marked the point at which sport, hitherto widely popular but unofficial in the armed services, became formally integrated into the military system, both as ‘recreational training’ and as an officially sanctioned form of leisure for other ranks. The British example was followed by other Allied forces – by the Dominion armies, by the United States and, despite considerable initial scepticism, by the French. The experience of the First World War had an enduring influence on the organisation and ideology of modern British military sport. When in 1931 General Harington declared that the war had been won by ‘leather’ in the shape of footballs and boxing gloves he was only expressing in exaggerated form the official recognition of sport’s military value. This chapter traces the process by which sport in the British army was transformed from a mainly spontaneous and improvised pastime in the early stages of the war into a compulsory activity for troops out of the line by the last months of the conflict. It examines the ways in which sport was seen to have military utility in improving fitness, relieving boredom, providing distraction from the horrors of war and building morale, officer–men relations and esprit de corps. Finally it demonstrates how the amateur model of sport, promoted energetically but largely unsuccessfully by army sports reformers before 1914, came to be imposed on service sport on the Western Front.