This Migration Observatory report examines the data available for understanding the EUSS (EU Settlement Scheme) for EU, EEA and Swiss Citizens and their family members living in the UK to secure their post-Brexit residence rights. It looks in detail at why the current data cannot tell us how many people have not yet applied to the scheme or which groups are more versus less likely to be doing so, and shows why for the official system around 3.4m non-Irish EU citizens is likely to be an under-estimate of the total eligible population, and other data gaps in our understanding of EUSS performance. Read the report.