The aim of this paper aims is to reconstruct the original context, the characteristics and the main assumptions of M. Hauriou’s theory of the institution as one of the most coherent and comprehensive examples of legal institutionalism. By means of critical source analysis, the paper presents the basic elements of the notion of the institution together with its ontological and epistemological assumptions concerning both the very concept of law and legal methodology. The second part of the paper presents the critical approach expressed in Hauriou’s theory towards some core concepts of public and constitutional law by opposition to traditional, normativist and positivist approaches to constitutionalism. Hauriou’s work, despite its importance as indirect inspiration for contemporary doctrines of constitutional pluralism, is nowadays barely present in European legal science discourse. The last part of the paper is therefore dedicated to some closing remarks on current relevance of the theory of the institution with particular focus on the challenges of increasing pluralism of legal orders.