Undocumented or unregistered immigrants are part of European society, particularly in Belgium. However, adequate and reliable data on this population is scarce as it is not always easy for data-collecting authorities to have access to and gather information on them. In this report, we estimate the number of undocumented migrants from Sub-Saharan and North Africa in Belgium based on mortality data from the Belgian statistical office. We first discuss the how mortality data has been used to study migrant populations and present a brief overview of existing knowledge on undocumented migrants in Belgium. We then look at mortality among regular or registered migrant populations in Belgium. In the last section, we extrapolate the number of undocumented migrants for the chosen regions. The report focusses on North African and Sub-Saharan African migrants as they constitute the largest undocumented population in Belgium. As per our estimation, the Sub-Saharan non-registered presence, including occasional visitors ...