The 42nd Infantry Division was an infantry division of the British Army. The division was raised in 1908 as part of the Territorial Force (TF), originally as the East Lancashire Division, and was redesignated as the 42nd Division on 25 May 1915. It was the first TF division to be sent overseas during the First World War. The division fought at Gallipoli, in the Sinai desert and on the Western Front in France and Belgium. Disbanded after the war, it was reformed in the Territorial Army (TA), in the Second World War it served as the 42nd Infantry Division with the British Expeditionary Force (BEF) and fought in Belgium and France before being evacuated at Dunkirk. The division was later reformed in the United Kingdom and, in November 1941, was converted into the 42nd Armoured Division, which was disbanded in October 1943 without serving overseas. A 2nd Line duplicate formation, the 66th Infantry Division, was created when the Territorials were doubled in both world wars.

The Territorial Force was a part-time volunteer component of the British Army, created in 1908 to augment British land forces without resorting to conscription. The new organisation consolidated the 19th-century Volunteer Force and yeomanry into a unified auxiliary, commanded by the War Office and administered by local county territorial associations. The Territorial Force was designed to reinforce the regular army in expeditionary operations abroad, but because of political opposition it was assigned to home defence. Members were liable for service anywhere in the UK and could not be compelled to serve overseas. In the first two months of the First World War, territorials volunteered for foreign service in significant numbers, allowing territorial units to be deployed abroad. They saw their first action on the Western Front during the initial German offensive of 1914, and the force filled the gap between the near destruction of the regular army that year and the arrival of the New Army in 1915. Territorial units were deployed to Gallipoli in 1915 and, following the failure of that campaign, provided the bulk of the British contribution to allied forces in the Sinai and Palestine Campaign. By the war's end, the Territorial Force had fielded twenty-three infantry divisions and two mounted divisions on foreign soil. It was demobilised after the war and reconstituted in 1921 as the Territorial Army.