In most political systems, the community of policy insiders represents a small subset of the total interest group population. Therefore, one key question is which factors explain why some mobilized interests become insiders and others remain outsiders. By contrasting a bottom-up registration of interest groups with a top-down census of all groups that enjoy access to policymakers, we present a unique approach to distinguish insiders from outsiders. This approach allows us to systematically analyze which factors-such as resources, constituency, scale of organization and policy portfolio-predict who becomes a policy insider. Our analysis focuses on interest group politics in Belgium, and shows that next to resources, the size of the membership, the scale of organization and a group's policy portfolio are strong predictors of the likelihood to gain access.