Due diligence has increasingly attracted the attention of scholars but it remains an elusive concept. This chapter explains why a study on the dogmatic foundations of due diligence obligations is timely. Thechapter argues that studies addressing the theory of public international law obligations are still scarce. Yet, they are crucial for properly understanding the externalities of obligations and their functioning on the international legal order. Starting with this premise, the chapter asserts that a theoretical study on due diligence obligations serves two main purposes. First, it allows to better clarify the nature, the content and the scope of operation of these obligations, which are traditionally considered difficult to pin down in the abstract. Second, the chapter advances the hypothesis, to be tested in the course of the book, that a dogmatic reconstruction of due diligence obligations also serves normative purposes. The introduction then sets the methodology and scope of the book and explains why the study of due diligence obligations will be limited to obligations of states and will not cover due diligence obligations of international organisations and non-state actors.