This study is the first volume of Cedefop research on empowering adults through upskilling and reskilling pathways. It estimates the magnitude of the low-skilled adult population according to a broader conceptualisation which goes beyond educational attainment levels, and considers digital skills, literacy and numeracy, as well as skill loss and skill obsolescence. The study also identifies different subgroups of adults most at risk of being low-skilled, by skill dimension. Better understanding the magnitude of the low-skilled adult population and recognising different and specific target groups within the heterogeneous group of adults with potential for upskilling and reskilling, is crucial to better design and implementation of effective upskilling pathways for those most in need. [For "Volume 2: Cedefop Analytical Framework for Developing Coordinated and Coherent Approaches to Upskilling Pathways for Low-Skilled Adults," see ED606955.]