This volume collects the lectures delivered during the seminar series "International Tax Fall on Tax Sustainability and Justice", held at the Faculty of Law of the University of Milan in Fall 2023, on the centenary of the League of Nations' Report on Double Taxation (1923-2023). The lectures, entrusted to scholars of recognised academic and institutional standing, provide a historical, criticai, and comparative analysis of the evolution of international tax law, with particular attention to the role of internatio-nal organisations (OECD, United Nations), tax cooperation, and recent developments in the fìeld of global minimum taxation.