The phrase «Massimo Severo Giannini as a constitutionalist» may refer to Giannini both as a constitutional jurist and as a protagonist of constitutional policy through various decades of the Italian Republic’s development. Given the vast field that these perspectives are likely to open, the author has chosen to limit his inquiry to the question of how Giannini conceived the relationship between the Constitution and constitutional law and its related implications for constitutionalists. In particular, the research aims to demonstrate that this relationship corresponds to that identified by Giannini’s mentor, Santi Romano, according to whom the Constitution was only a written text, while constitutional law covered the entire field concerning the State. However, while Romano’s distinction was driven by the purpose of advocating for the foundational character of the old conception of State sovereignty vis-à-vis any constitutional text, Giannini, who had himself taken part in drafting the 1948 Constitution, tended rather to support the view that studying the State’s transformations was crucial for an understanding not only of constitutional law but also of the actual meaning of the Constitution.