Many philosophers regard collective behavior and attitudes as the ground of the whole of social reality. According to this popular view, society is composed basically of collective intentions and cooperative behaviors; this is so both for informal contexts involving small groups and for complex institutional structures. In this article, I challenge this view, and propose an alternative approach, which I term institutional externalism. I argue that institutions are characterized by the tendency to defer to elements that are external to the content of collective intentions—such as laws, declarations, and contracts. According to institutional externalism, those elements are the grounds of institutional statutes, rights, and duties.