Constitutions are commonly regarded as uniquely national products, shaped by domestic ideals and politics. This paper develops and empirically tests a novel hypothesis, which is that constitutions are also shaped by transnational influence, or “diffusion.” Constitutional rights can diffuse through four mechanisms: coercion, competition, learning and acculturation. To test diffusion in the constitutional realm, we traced the historical documents of all post-WWII constitutions and documented the presence of 108 constitutional rights. Using a sample of these rights in 180 countries between 1948 and 2001, we estimate a spatial lag model to explain their adoption. Our results show that countries follow the choices of their former colonizer, countries with the same legal origin, the same religion, the same former colonizer, and the same aid donor. These transnational influences are strongest when a nation adopts its first constitution. At this time, no less than 46 percent of the variation in a bill of rights results from transnational influences.