The specific features of obligations erga omnes cannot be fully appreciated if the concept is interpreted in isolation. Part I of the present study intends to avoid this problem by addressing a number of basic issues. It proceeds in two steps. Chapter 1 addresses fundamental legal concepts; its aim is to clarify the basic parameters, within which the subsequent analysis evolves. Chapter 2 presents the historical background by providing an overview over the traditional, pre-Barcelona Traction rules of law enforcement. Taken together, both chapters are intended to set the stage for the subsequent discussion of specific problems raised by the erga omnes concept.