This chapter explores the foundations of due diligence under international law. Due diligence emerged in the international practice of the nineteenth century concerning diplomatic protection and the security of states, and developed as a notion linked to the responsibility of states in connection with acts of private individuals. For a long time, due diligence was conceived as a concept pertaining to the realm of international responsibility and it was primarily associated with the measure of a state organ’s fault. The chapter illustrates how, during the twentieth century, due diligence migrated from the realm of secondary rules to primary rules. The chapter clarifies the relationship between due diligence and overlapping concepts, like international liability and the notion of general principles of international law. It is argued that due diligence should be construed as an identifier for a typology of international obligations, something that provides meaning and rationale to them. The chapter concludes by clarifying the difference between due diligence as a ‘qualifier’ for primary rules of states, and due diligence as a ‘process’ linked to the activities of non-state actors.