This was previously published as a special issue of Journal of Ethnic & Migration Studies. How do states in Western Europe deal with the challenges of migration for citizenship? The legal relationship between a person and a state becomes increasingly blurred in a mobile and transnational world. This volume deals with the membership dimension of citizenship and specifically with the formal rules that states use to attribute citizenship. These nationally specific rules determine how and under which conditions citizenship is attributed by states to individuals: how one can acquire formal citizenship status, but also how it can be lost. We observe six trends in citizenship policies since the early 1980s. First, we observe a trend toward completing the equal treatment of women and men with regard to descent-based citizenship attribution. Second, there is a process of convergence between countries with ius soli and ius sanguinis traditions with regard to birthright provisions. Third, the increasing acceptance of multiple citizenship is reflected in a dual trend to abolish or moderate the renunciation of the citizenship of origin as a condition for naturalisation and also to abolish or restrict provisions of loss of citizenship due to voluntary acquisition of a foreign citizenship. Fourth, many countries have introduced language tests and integration conditions in the naturalisation procedure and some countries are now also concluding the naturalisation process by means of a US-styled citizenship ceremony. Fifth, states increasingly take the principle of avoiding statelessness into account into their citizenship laws. Finally, we see that states start to take membership of the European Union into account in their citizenship laws. Chapters in this volume discuss both these broad trends across Western Europe, analyzing historical patterns and recent change, as well as specific developments in individual countries.