In this introduction, we lay out the premises, logics and content of the book in more detail. In the next section, we introduce a varied set of current international challenges concerning the relationship between states and citizens. In the third section, we present the historical background for why states are interested in citizens beyond the border, and the different forms this interest has taken over the centuries. This feeds into the discussion about the contemporary understanding and practice of the Duty of Care in the fourth and fifth sections. Here we discuss how the concept allows for new insights into current topics, as well as how it reconfigures and ties together insights from existing literatures. In the sixth and final section, we specify how one can go about studying the Duty of Care, with reference to the ensuing chapters of the book. In this section, we emphasise the chains of care, the power relations inherent in them and the dilemmas and paradoxes that arise from asserting and claiming a Duty of Care.