Constitutions are often perceived as national products. This paper develops and empirically tests a different hypothesis, which is that constitutions are also externally driven, or shaped through transnational influence or diffusion. Constitutional norms can diffuse through four possible mechanisms: competition, coercion, learning and acculturation. Using a new panel dataset based on our coding of the constitutions of 192 countries between 1946 and 2006, we estimate a spatial lag model to explain the adoption of constitutional provisions. We find evidence of diffusion among countries with the same legal origin, the same religion, and the same former colonizer. Such patterns indicate that constitution-makers learn from, or simply emulate, the constitutional arrangements of states with similar cultural and legal backgrounds. But we also find that constitution-making is affected by the constitutional choices of former colonizers and by foreign aid flows. These findings suggest that the carrots and sticks of powerful states are an important force behind constitutional design around the world.