This paper studies the founding concepts of Carl Schmitt's constitutional theory, as conceived in Constitutional Theory (1928), in light of its institutional turn. To this end, it first presents Schmitt's decisionism, as expressed in his main writings of the 1920s, including Constitutional Theory. In second place, it exhibits the institutional turn that his work underwent in the 1930s, which implied the -partial- abandonment of his decisionist proposal. Afterwards, it offers an analysis of the categories of Schmittian constitutional theory in light of his institutional conceptual apparatus. It is concluded that these concepts, as formulated in Constitutional Theory, are not compatible with his institutional proposal, which requires their reconceptualization.