Arguably no idea has been more central to democratic government than the separation of powers. In this seminal book, Prof. Giovanni Bognetti (1930- 2013), traced the history and the evolution of this foundational idea in modern constitutionalism. Prof. Bognetti distinguishes two models of separation of powers: the “classic” model, emerged in reaction to the centralization of powers typical of absolutist states as an effort to protect individual liberties and the “social” model, which reflects the new paradigm of social rights protection in modern democracies. In the latter, according to Prof. Bognetti, we perceive a kind of political and legal transformation of the classic conception of separation of powers—a transformation that becomes even more pronounced against the backdrop of globalization, multiculturalism and the rise of supranational legal orders. This seminal book represents a fundamental contribution to the current global debate on the separation of powers.