viii Preface have become a threat. These practices have witnessed a recent comeback in liberal democracies that deprive terrorist suspects of their nationality in order to make them deportable or to prevent them from returning. How concerned should we be about this trend given the small numbers of affected individuals and the severity of the threat they pose? Does it signal an erosion of the idea that liberal states may control admission to citizenship but should not have the power to deprive citizens of their nationality – abdicating thereby also their responsibility to protect or to punish them? The third and fourth debates look more explicitly towards the future and consider how changing social norms and new technologies may affect the substance and salience of national citizenship. National citizenship is always acquired by birth and all states use a principle of ius sanguinis. In the large majority of countries, citizenship is bestowed upon those born to citizen parents independently of their country of birth. Ius soli, i.e. citizenship acquired by birth in the territory is the dominant principle only in the Americas, but even there second generations born abroad are citizens by descent. The debate on the future of ius sanguinis asks how this most basic and widespread rule for citizenship attribution is going to be affected by new patterns of family relations and new reproductive technologies. Will biological descent continue to count for more than social relations of children to their caregivers? And will these changes provide opportunities for abandoning ius sanguinis altogether and replacing it with territorial citizenship based on birth and residence in a country? The fourth and last debate challenges this possible conclusion by pointing to another major transformation of contemporary societies: the digital revolution that offers also new opportunities for global citizenship. Can blockchain technology provide every human being with a single global legal persona and will it permit the creation of political communities without territory? Would such developments complement or replace the existing functions of citizenship in territorial states? Or will the latter instead capture new digital technologies in order to enhance their control over society? The debates collected in this volume follow certain rules. The texts are not freestanding submissions to a blog or edited volume. Each author was invited to respond not only to the introductory kick-off essay but also to the comments previously published online. Authors were invited based on their expertise and stances on the topic, with the aim of representing a broad spectrum of reasonable views. This unusual format avoids repetition and simulates a conversation among people who disagree with each other but are ready to listen and address each other’s arguments.