What legitimates constitutions? One standard answer is that constitutions are legitimate only if they represent the people they govern. This article identifies two different conceptions of representation. Representation can be grounded either in the consent or the will of the citizens or when the constitution reflects the ‘real’ identity of the members of the nation. Alternatively, it is sometimes stated that the constitution is legitimate because it promotes justice or, more generally, is grounded in reason. While constitutions are typically grounded both in claims to represent the people and in claims concerning the justness and wisdom of the constitutional provisions, we establish that there are two types of constitutions: constitutions that are primarily representational (e.g. the US Constitution) and constitutions that are primarily reason-based (e.g. the German Constitution). We also show that this distinction has important ramifications for how constitutions are drafted and ratified, and how they operate. One central implication is that the legitimacy of constitutions that make weak claims to representation – for example, constitutions that are imposed by foreign powers – can still be defended on reason-based grounds.