Based on mixed-methods research and ethnographic fieldwork at various sites in Italy, this book examines the relationship between expertise and activism in grassroots environmentalism. Presenting interviews with citizens, activists and experts, it considers activism surrounding infrastructure in urban areas, in connection with transport, tourism and waste disposal. Through comparisons between the sites, the author analyses the ways in which citizens, political activists and technical experts participate in using expertise, shedding light on the effects of this on the structure and composition of social movements, as well as the implications for the mechanisms of participation and the formation of alliances. Bridging the sociology of expertise and contentious politics, this study of the relationship between contentious expertise and democratic accountability shows how conflict transforms, rather than inhibits, expertise production into a ‘contentious politics by other means’. As such, it will appeal to social scientists with interests in social movements, environmental sociology, science and technology studies, and the sociology of knowledge.