Abstract: How is constitutionalism possible, when people disagree on so many questions about what is good and what is right? This essay, written for a special issue of Social Research on Difficult Decisions, explores the role of two kinds of incompletely theorized agreements amidst sharp disagreements about the largest issues in social life. The first consist of agreements on abstract formulations (freedom of speech, equality under the law); these agreements are crucial to constitution-making as a social practice. The second consist of agreements on particular doctrines and practices; these agreements are crucial to life and law under existing constitutions. Incompletely theorized agreements help illuminate an enduring constitutional puzzle: how members of diverse societies can work together on terms of mutual respect amidst intense disagreements about both the right and the good. Such agreements help make constitutions and constitutional law possible, even within nations whose citizens cannot concur on the most fundamental matters.