One of the dilemmas facing multi-ethnic societies is how to recognize ethnic diversity while avoiding the risk of reifying ethnic identities, i.e. conceiving ethnic groups as homogeneous and static entities. Ethnic categorization is an essential and necessary tool for developing adequate policies to address unequal opportunities and discrimination. At the same time, ethnic categorizationis an inherently problematic procedure, in official statistics but also in ethnic and migration studies. In fact, ethnic categorization may involuntarily contribute to the reinforcement of stereotypes and negative attitudes towards immigrant groups and, by and large, to the ‘racialization’ of society. The aim of this issue brief is two-fold: (1) to present and compare how the issues of ethnic categorization are dealt with by researchers of selected EU countries and (2) to highlight how cross-country differences in ethnic categorization and pan-ethnic classification practices are also related to different population-counting traditions.