During the Mandate period, the struggle for Palestine was essentially a demographic race between the Jewish minority and the Arab majority, with the Mandate authorities determining the rules of the game. While the proportion of Arabs to Jews at the end of WWI was 11:1, by the eve of WWII, it was approximately two-thirds Arabs to one-third Jews, and remained as such until the outbreak of the 1948 War, with 600,000 Jews in the country and twice as many Arabs. The primary source of growth in the Jewish population was immigration whereas the rate of growth among the Arabs was due almost exclusively to natural population increase. The article surveys and analyzes the role of demography in shaping the policy and practice of the three sides of the Palestine triangle from the formulation of the Balfour declaration in 1917 to the 1947 United Nations’ partition resolution. The main contention is, that demographic calculations and estimations were behind the positions on the three main issues around which the conflict in Mandatory Palestine revolved: immigration, the establishment of institutions of representative self-government and the acquisition of land by Jews.