In recent decades, there has been a wide-ranging global movement towards constitutional review. This development poses important puzzles of political economy: Why would self-interested governments willingly constrain themselves by constitutional means? What explains the global shift towards judicial supremacy? Though different theories have been proposed, none have been systematically tested against each other using quantitative empirical methods. In this paper we utilize a unique new dataset on constitutional review for 204 countries for the period 1781-2011 to test various theories that explain the adoption of constitutional review. Using a fixed effects spatial lag model, we find substantial evidence that the adoption of constitutional review is driven by domestic electoral politics. By contrast, we find no general evidence that constitutional review adoption results from ideational factors, federalism, or international norm diffusion.