Since the concept of due diligence first appeared in arbitral decisions at the end of the nineteenth century, its success in international law has been growing steadily. Yet its nature, sources and regime remain indeterminate in many ways. In response to the objections currently raised against it, this book provides a critical analysis of the practice of due diligence in international law. Its aim is to determine whether a principle, standard and/or obligation of due diligence does indeed exist under general international law; to identify what could constitute its normative structure, foundation and general regime; to establish the conditions, content and modalities of implementation of international responsibility for negligence; and, finally, to examine the specificities of due diligence in special regimes of international law, such as international environmental law, international cybersecurity law and international human rights law. More generally, the book also examines the reasons for due diligence's "renaissance" in international law's recent history and explains what this revival says about the state of the international legal and institutional order and the possibilities for its reform.