Volunteers eager to help disaster victims have begun to draw on open source models of organization to mobilize and coordinate vast resources from around the world. This paper investigates two such groundbreaking efforts, involving responses to Hurricane Katrina and to the South East Asian tsunami. The study sheds light on how these organizations evolve so rapidly, how leaders emerge and confront challenges, and how interactions with traditional, more hierarchical disaster recovery efforts unfold. Lessons from these early efforts show how they can be improved, and also point to the need for more research on networked non–state actors that are playing increasingly prominent roles.