The idea that there are some norms that, because of their content, have a fundamental character and should be treated as such by the legal system has a long trajectory in the history of constitutional thought. In the 19th century, it was usually expressed through the notion of the internal (or historical) constitution. The better known of these is the concept of the constitution in the material sense, generally understood as including the norms that establish the basic structure of the state and that regulate the legal relations between state and citizens. This paper explores the notion of the material constitution in the work of several mid-20th century constitutional theorists. The objective will not be to summarise the constitutional thought of these authors but to show the ways in which they understood the relationship between the material constitution and constituent power, and how that understanding affected their views about the limits of constitutional reform.