This policy brief provides a concise summary of information that can help policymakers and other stakeholders grasp the main issues around local-level estimates of irregular migrant populations and make decisions about which factors to prioritise while making these estimates. Drawing on examples from municipalities across Europe and Canada, we highlight the scarcity of official local-level estimates, how the divergences in terminology feed into the uncertainty around estimates, the challenges in collecting local-level data as well as showcasing promising practices. Whilst local-level estimates are rare, they can be a way towards overcoming some of the limitations of national estimates including data sources that may not be available nationally, such as the numbers of irregular migrants accessing specific local services and can help inform the development of local-level policy and practice. However just as with national estimates, local-level estimates may pose ethical issues in how they are used and for which purpose.