Given the widespread interest in political solutions to the current problems associated with immigration, we need to have an accurate understanding of existing policies in a cross-national perspective. To explain the coming into being and effectiveness of these policies, researchers have recently started to quantify immigration and citizenship policies and built databases across time and a large number of countries. These indices are likely to reconnect political science research with a field from which it has long been disconnected in terms of theories and methodology—the sub-field of migration and citizenship research. This special issue brings together scholars from North America and Europe who have been at the forefront of index-building and have started to employ these indices in empirical research.