Since the 1972 Stockholm Conference on the Human Environment, ecological pressures on our planet have grown more acute. Yet, modern environmental law has also continued to evolve and spread within international as well as among national legal systems. With the paths of international and national environmental law becoming increasingly intertwined over the years, international environmental legal norms and principles are now penetrating deeper into national legal systems, and environmental treaties are increasingly incorporating or referencing national legal norms and practices. The shifting legal landscape is also changing contemporary environmental law practice, creating greater needs for domestic environmental lawyers to be informed by international law and vice versa. This essay describes how domestic environmental law practice is increasingly informed by international legal norms, while the effective practice of international environmental law more and more requires enhanced awareness, and even understanding, of national environmental regulatory and governance systems. It illustrates these trends with the historical role and work of the United States Environmental Protection Agency’s Office of General Counsel.