As the title of the work indicates, we are united by the shared desire to understand, from a comparative perspective, the change in the government of the Royal Treasury of the Spanish Monarchy and the French Monarchy in the 18th century, a century in which that both crowns shared the same reigning house. Since the end of the 20th century, the historiography relating to the experiences of the two monarchies in this period has modified the portrait of both, which we fully account for in each chapter. However, the specific and limited analysis of various aspects of the problem of the government of these tax entities has generated knowledge that, we consider, is convenient to synthesize and compare in order to achieve a long-term vision that integrates them and allows them to be reevaluated. With this purpose, the book privileges the study of the activity of organizations and actors - their structures, their reform projects, their representations of good government and the licit uses of money, their strategies, the resistance they faced - to examine how they gave life and meaning to the institutions of the royal treasuries. In other words, this history is at the same time political, economic and cultural. In it, institutions are not conceived as immutable structures unrelated to the actions of groups and individuals, but as the result, often contradictory and uncertain, of said actions. That is what this book is about.