Key points • Analysing irregular migrant stocks using register or other administrative data can prove challenging given the usually undocumented nature of the phenomenon; yet, some recent efforts highlight the potential of register data. • The German Central Register of Foreigners (AZR) provides longitudinal data on non- Germans staying or having previously stayed in the country, including subsets of irregularly staying migrants, allowing for in-depth analyses of various research questions. • Throughout, the chapter provides real-life examples of how register data has been used in irregular migration research across different countries. Testo Capitolo di libro 2025-09-30 doi:10.48341/g31s-vq79-07 eng Register data sources on migrant stocks Duldung http://creativecommons.org/licenses/by-nc/4.0/ Text book part Laura Peitz University of Krems Press Text Buchkapitel Register data administrative sources migrants under an obligation to leave toleration Irregular migrant stocks Data integration AZR Germany application/pdf https://door.donau-uni.ac.at/o:5687