On September 25, 1942, the government of Sweden ordered a census of Roma and Travellers in the country. The mapping of these groups was to serve as a first step towards solving the perceived Gypsy problem. The census did not proceed smoothly, mainly because of conflicts within the scholarly community. On the basis of studies undertaken in fully sovereign Sweden during the World War II period, the author of this article clarifies the role experts played in the scientific legitimization of the registration process.