This article offers the first detailed history of the norm of treaty publication as it has evolved over the last century. Drawing on both public debates and archives of foreign ministries, it traces how, and why, secret treaties have persisted, even in liberal democracies. It challenges assumptions of ever-greater transparency over time, and complicates the associations made—by interwar reformers and international lawyers today—between the norm of treaty publication and ideals of legality in the international order.