This book is a historical and comparative introduction to public law, defined as the law of the res publica (the "public thing" or the enduring common interests of a people). It traces back the origins of the res publica to Roman law, and it analyzes the course of its development, first, during the monarchical age in continental Europe and England, and then, during the republican age that began at the end of the eighteenth century with the democratic revolutions in the United States and France. For each period and country, the book analyzes the major concepts of public law and their transformations: sovereignty, the state, the statute, the separation of powers, the public interest, and administrative justice.