The image of citizens (identity) plays a key role in citizen and government relationships and identifiability is perceived relevant in many contexts of public administration. This is particularly the case where citizens can exercise rights and claim benefits, and for various purposes, like administration and registration, public safety, security, general law enforcement, policy-making, citizen mobility, and, more generally, to exercise control. Citizen identities as constructed and maintained by government are changing in the information age: they become ever richer and more complete. Information from different sources is linked together to form comprehensive accounts of individuals. Simple records pertaining to clients or citizens are increasingly enriched by other data to form more complete pictures of the individuals involved. Up to now, this mainly concerned data provided by the individuals (the data subjects) themselves, or on the interpretation or decision of the record keepers (the data controllers). More and more personal data is and will be processed from other sources, as a result of new technologies and applications, like Radio Frequency Identification, biometrics, Location-Based Services, profiling, data mining and ubiquitous computing. This trend includes the processing of relatively new kinds of personal data, such as location and behavioural data, and the identifiability of more objects and persons because of an increased use of unique identifying numbers. Additionally, personal data is stored more extensively, for example, as a result of camera surveillance, Internet use and Digital Rights Management. Moreover, data becomes increasingly accessible as a result of digitalisation, linking and automatic recognition as well as technology convergence and tracking and tracing (behaviour of) individuals from a distance. Increased personal-data intensity and sophisticated methods of analysing data, like data mining, lead to more complex identities of individuals. What these identities amount to is relatively unclear, as is their impact on policy making, execution and citizens. This book provides a conceptual framework for ‘identity’ and other related concepts, and explores, both theoretically and empirically, how socio-technological, policy and legal developments change the construction of citizen identities and, hence, citizen and government relations.