Massive twentieth century immigration in Belgium led to the emergence of a profoundly diverse society. However, until the 1980s, since a particularly restrictive nationality law was in force, it was believed that nationality-based distinctions adequately addressed the issue. But since 1984, the many reforms to the Code of Belgian citizenship have strongly challenged the consensus. In fact, Belgian nationality became one of the simplest national statuses to acquire in Europe, and nationality-based statistics reflect the action for nationality law and the consequences of the waves of immigration. Wedged between France and the Netherlands – two countries with radically different approaches to understanding diversity – Belgium has made the ethnic reference a taboo, especially in French-speaking regions. However, the need to grasp the situation of foreign-born populations seems to be slowly gaining momentum from a scientific and political perspective, especially with regards to the nation’s response to discrimination. Despite the marked divergences that still exist, the nation seems to be developing statistics on national origin based on objective criteria such as the place of birth and nationality of an individual and his/her ascendants, and, even though it is not used, this information is recorded by the extended administrative records system. This chapter presents the methodological choices that are emerging despite continuing opposition and their implications.