This book offers a first overarching look at the relationship between states and their citizens abroad, approached through the concept "duty of care". How can society best be protected, when increasing numbers of citizens are found outside the borders of the state? With over 1.2 billion tourists crossing borders every day and more than 230 million expatriates, questions over the sort of duty states have for citizens abroad are politically pressing. Contributors explore both theoretical topics and empirical case-studies, examining issues such as as how to care for citizens who become embroiled in political or humanitarian crises while travelling, and exploring what rights and duties states should acknowledge toward nationals who have opted to take up arms for terrorist organizations. This work will be of great interest to scholars in a wide range of academic fields including international relations, international security, peacebuilding, ethics and migration.