A core function of constitutions, unwritten and written alike, is to constrain the actions of government officials. This chapter argues that a constitution cannot serve this function unless communities of “constitutional participants” pay attention to and engage with the constitution and its meaning. Participants are needed not only to develop shared understandings of ambiguous text but also to create a credible threat of sanctions, legal or otherwise, for unconstitutional actions. The chapter further suggests that American states may have, at least on certain issues, constitutional communities too sparse to activate constitutional constraint. Finally, the chapter briefly problematizes the creation of constitutional communities, asking whether and how they could be forged in the states without generating costs of capture or further polarization.