Identification practices such as population and address registers, identity cards, biometric visas and passports, and the joining up of administrative data are rapidly becoming part of a new regime of government practices in the EU. The paper investigates how these practices are being advanced in the UK for the purposes of constructing and knowing whole populations as objects of management and government. The argument is developed that the knowledge of subjects compiled by identification practices constitutes a new numbers regime called ‘population metrics,’ made up of different measurements of subjects such as biographical, biometric and transactional data. Population metrics is also used to refer to the practice of combining, matching, assembling and reassembling metrics to identify ‘new’ populations. While identification practices raise concerns about privacy or surveillance, population metrics bring to the fore their totalizing effects, and raises questions about how we are known and governed. The paper poses some of these questions and suggests that this regime of metrics opens up a new politics of population.