The objective of this working paper is to provide a methodological assessment of available data on irregular migration flows in Europe and other contexts. It is aimed at the public, including those who may not have a technical understanding of migration statistics. It also serves as a companion document to the forthcoming MIrreM Public Database on Irregular Migrant Flow Estimates and Indicators (MIrreM D5.2).[1] In MIrreM, irregular migration is operationally defined as a form of migration that is not “regular,” “unlawful,” or not according to the rules (Kraler & Ahrens, 2023). The paper addresses this research question: How are irregular migration flows measured and estimated in the EU and selected other countries and what can be said about the quality of the available indicators and estimates? [1] Siruno, L., Leerkes, A., Badre, A., Bircan, T., Brunovská, E., Cacciapaglia, M., Carvalho, J., Cassain, L., Cyrus, N., Desmond, A., Fihel, A., Finotelli, C., Ghio, D., Hendow, M., Heylin, R., Jauhiainen, J.S., Jovanovic, K., Kierans, D., Mohan, S.S., Nikolova, M., Oruc, N., Ramos, M.P.G., Rössl, L., Sağiroğlu, A.Z., Santos, S., Schütze, T., & Sohst, R.R. (2024). MIrreM Public Database on Irregular Migration Flow Estimates and Indicators. Krems: University for Continuing Education Krems (Danube University Krems). https://doi.org/10.5281/zenodo.10813413