Maarten AldersPaper presented at the conference "The measure and mismeasure of population. The statistical use of ethnic and racial categories in multicultural societies." in Paris, France, 17 - 18 december 2001.This paper describes the classification of the population with a foreign background in the Netherlands. The classification is based on a conceptual definition which distinguishes a first and a second generation. The first generation consists of persons who are born abroad and have at least one parent who is also born abroad. The second generation consists of persons who are born in the Netherlands and have at least one parent belonging to the first generation.