Exclusion from social protection remains a significant challenge. This study, based on interviews with researchers and programme implementers in Indonesia, Kenya, South Africa and Uganda, and a literature review and experience across many countries, explores factors behind this. To date, most analysis of exclusion has been quantitative, focusing on measuring targeting errors. As a result, little is known about the causes of exclusion, including the impact of underlying social and political processes and dynamics. This report aims to stimulate debate on the causes of exclusion and recommends potential strategies to address this.