Since the mid-1930s, theNazi regime concerned itself with the systematic registration and identificationof Roma. At the 1935 Copenhagen Interpol Conference participating states backedthe initiative proposed by the German police regarding the creation of aninternational registry of Roma. It had been easier to classify Jews for recordsheld by religious communities were readily available to the state. Many Roma inEurope were nomadic and ID-less. The study focuses on measures ofidentification and registration of Roma undertaken in sovereign Sweden and therole of experts and census takers in transnational context. On 25 September1942, the government of Sweden ordered inventory of Roma and Travellers. Thepurpose of the registration was to solve “a problem” by mapping both thesegroups. In Sweden the census did not proceed smoothly, because of the conflictswithin the experts’ community. The paper focuses on the transnational studiesof registration of Roma undertaken in the fully-sovereign Sweden and the roleof experts in ‘scientific’ legitimation of this process.