Although health-based social movements organized by grassroots activists have a rich history in impacting health and social policy, few systematic studies have addressed their policy change efforts or effectiveness. In this article, the authors trace how four health-based social movements-the women's health movement, ACT UP, breast cancer, and needle exchange-influenced health and social policy legislation. The activists' efforts wrested control of "authoritative knowledge" that had once been the sole domain of "experts" with advanced medical training. They used this knowledge to empower "average" people with medical information, promote self help and engage in civil disobedience, which led to changes in healthcare delivery, drug testing and approval, and increased research funds for HIV/AIDS, breast cancer, and needle exchange. The activists' efforts led to other health-based social movements that are currently, or will become, issues for health and social policy analysts in the future.