## 1 Introduction
### Population Registers  
A population register is an inventory of residents within a country, with their characteristics (date of birth, sex, marital status, _etc._) and other socio-economic data, for instance occupation or education. According to the United Nations Statistics Division's definition [2], \"the main administrative functions of population registers are to provide reliable information for the various purposes of government, particularly for program planning, budgeting and taxation; for issuing unique personal identification numbers; for establishing the eligibility of individuals for voting, education, health, military service, social insurance and welfare and the pension system; and for police and judicial references. Population registers are also useful for population estimation, census planning, and census evaluation and for sampling frame of household surveys.\" In addition to _stricto sensu_ population registers, there are many other public registers or official sources that store data on citizens: Tax, land, building and housing, military, foreigners, insurance, debt collection, welfare, courts, retirement, vehicles, and so on. All over Europe public registers are changing from paper-based files to digitalized registers and from decentralized registers to interoperable registers, although situations differ strongly from country to country. Moreover in the last decades many European countries have adopted laws on transparency and access to public information on the one side and laws preserving the privacy rights of individuals on the other. These laws apply to public register data as well, but at the moment there is limited questioning regarding the strategies to be developed: are data sharing and data protection compatible, should tension mitigation processes be set up to find a balance between data sharing and data protection in a given context and should data sharing or data protection prevail? Data from population registers are subject to tensions between (notably) security, privacy, interoperability, usability and intrusiveness, and experience in using such data is still lacking in most European countries. The situation of public registers and privacy policies varies considerably from country to country. It is very difficult to present a global view of the situation in Europe, but let us briefly present some representative examples. Finland has a long history in the field [3] and was amongst the first European countries to collect data on population, during the sixteenth century; census lists (local population registers) have been maintained in Finland since 1634. Finland moreover created a centralized population register by law in 1969 and the computer-based register was introduced in 1971, also pioneering in that domain. Finnish citizens may have access to the central population registers and can request the authority to correct their data. Last the Finnish eID card, introduced in 1999, was the first ever operational national eID scheme. In the United Kingdom, mandatory civil registration of births, marriages and deaths was first introduced in 1837 [4]. The United Kingdom still has decentralized paper-based civil registers; the administration of individual registration districts is the responsibility of registrars in the relevant local authority. Official registers are not directly accessible by the general public; however indexes are made available that can be used to find relevant register entries. Certified copies of the entries are issued for genealogical research or for administrative purposes such as passport applications. Some local indexesare published on the Internet. The UK does not have identity cards (although they are being piloted on a voluntary basis) and the proposal to introduce them is politically highly controversial. The Kingdom of Belgium conducted a first general population census in 1846.