Investigations of the Armenian Genocide began as soon as news of the mass killings reached Great Britain in May 1915. The 1916 publication in Britain of The Treatment of the Armenians in the Ottoman Empire as a Parliamentary Blue Book by Viscount James Bryce provided compelling and verifiable evidence of the systematic elimination of the Armenian population of the Ottoman Empire. Today the Blue Book is largely forgotten as a documentary and historical source on the massacres. This article situates the Blue Book in its historical context, arguing that its publication provided the foundation for the evidentiary standard by which the act of genocide is corroborated and judged by the international community. The uses and abuses of the Blue Book at the time by the British government and the humanitarian movement in making the case for genocide reveal the problematic legacy of the first efforts to catalog and document what the Allies labeled during World War I as “crimes against humanity.”