The widespread adoption of written constitutions is one of the most notable developments in institutional design in politics over the past 250 years. The American states offer a rich place to study constitutional innovation and imitation as being among the first political bodies to adopt constitutions and also given that they often replaced them, in both cases innovating and learning from one another. In this paper, we use quantitative text analysis to identify constitutional innovation and to investigate patterns of imitation. First, we find substantial textual borrowing between state constitutions. On average, 20 percent of a state’s constitutional language was borrowed directly from another state constitution. Second, states were more likely to borrow text from geographically proximate states, from temporally proximate state constitutions, and from states that shared similar partisan profiles. Finally, we offer a brief discussion of the most influential constitutions as an exploratory example for extending our approach of identifying textual innovation and imitation. These findings offer new contributions to both the study of constitutional design and institutional diffusion.