Social closure is a process through which some groups, implicitly or explicitly, draw categorical boundaries around themselves and others to monopolize resources. Social closure has two faces: opportunity-hoarding for actors’ categorical in-group and exclusion of the out-group. We explore closure case studies around criminal records, occupational licensing, education, non-compete employment contracts, job segregation, sexual harassment, access to science and technology jobs, and discrimination based on race, gender, sexual orientation, and religion. The case studies also highlight the important role of organizational and institutional variation in the degree and incidence of closure processes. We conclude that closure processes can be challenged by usurpationary movements, institutional regulation, and interactional resistance.