Grassroots environmentalism depends on the passion and commitment of local activists, most of them volunteers, who donate their time, skills, and other resources to efforts to protect their neighborhoods and communities as well as public lands. It also requires some type of organization to bring community members together, as well as strategies and tactics, both to accomplish goals and to build the movement by engaging supporters in collective action. This chapter begins to explain the mobilization of grassroots environmentalism by showing how compelling environmental issues create concerns among local residents and lead many to become active. It also describes how movement actors work to frame issues in ways that capture the experiences and grievances of potential constituents. Chapters 5 and 6 then continue the story of how grassroots environmentalists mobilize by looking at how concerned individuals become movement participants and how movement structures and communities continue to organize them. To explain the process of mobilization, I draw extensively on my observations of the anti-fracking activities and campaigns that took place during the time of my study.