This introductory chapter provides an overview of the twenty-first-century global security environment. Since the turn of the century, the global security environment has become increasingly dynamic, complex, and volatile; and the causes, mechanisms, and consequences of national and international (in-)security have become increasingly transnational and global in nature. Various powerful dynamics of a geopolitical, demographic, climatic, technological, social, and economic nature have been driving this trend, which has now been taken to entirely new levels by the recent outbreak of the COVID-19 pandemic. The convergence of so-called ‘old and new security challenges’, such as the return to power politics, the rise of asymmetric and hybrid warfare, and the emergence of novel threats posed by potent non-State actors, technological innovation, as well as dramatically increased economic, pandemic, and environmental risks, have entailed a veritable globalization of the security agenda. The chapter then outlines a number of overarching key dynamics, trends, and contestations that reflect the various intrinsic and extrinsic pressures and tensions international law is exposed to in the global security environment of the twenty-first century.