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 and visa-holders amounted to about 27,500individuals if it was counted on a random day between 2012 and 2016. For North Africans during the same period, the numbers amount to about 50,000. This would mean that over 1 in 3North African nationals encountered in Belgium would not be a registered resident. In sum, we estimate the total number of non-registered inhabitants of Sub-Saharan African and North-African origin living in Belgium between 2012 and 2016 around 80,000people. This is less than 1% of the total population of Belgium.