This article aims to clarify the scope of questions about religion in population censuses, and attempts to explain why such questions were included or left out of censuses taken in different nations and periods. The quantitative aspect is a fundamental question for students of religion interested in knowing where it is possible to rely on statistics about the size of confessional groups and their basic characteristics. A common use of the census in connection with religion has been to create aggregates about the size of different congregations by nation, and to cross-tabulate this with other variables such as gender, occupations, ethnicity, or regions. Enumerations with questions about religion were performed in many countries from the mid-nineteenth century, but questions about religious affiliation never entered the US censuses, and were left out of most censuses in many other countries as is indicated in the map in figure 1. We shall try to clarify how pressure was put on statistical bureaus, parliaments, and governments to promote or hinder the inclusion of questions about religion.