Urban Action Networks is a study of how communities organize in response to threats to their lives and well being. As HIV/AIDS wreaked havoc on the worlds of some of the most marginal and disenfranchised people in New York, they came together to create a shared response, forming a new organizational field within which their various efforts were coordinated. How the communities of the most affected people organized, reorganized, and redefined the social and political context of HIV/AIDS offers an encouraging glimpse into the way in which marginal communities can convert shared needs into collective action.