Mobilizations on high-tech projects often become arenas of contention where expertise crosses political and technical claims. One of the aspects of these citizen mobilizations resides in the elaboration of alternative politics linking bottom-up communitarian knowledge with expert advice. This innovation addresses important questions for participation and democracy in general, since expert knowledge indeed maintains a delicate relationship with democratic politics. In this work I aim to analyze how common citizens, political activists and technical experts participate in using expertise, while contributing to making «technical democracy» work. Starting from a dataset of more than 500 episodes of contention regarding high-tech projects, I focus on an in-depth comparative study of mobilizations in the cities of Venice and Florence, given their importance in the rise of the so called «new environmentalism» in Italy. Analyzing four protest campaigns I shed light on the mechanisms of co-production. focusing on 1) the characteristics of bottom-up citizens’ expertise, 2) experts’ enrollment and their peculiar forms of engagement. In both cities I have selected two cases depending on their variation in terms of technological complexity, conflict intensity and citizens' participation. While in some high-tech projects political conflict and technical controversy tend to be confined to restricted mobilizations – regarding mainly activists and experts – others show high levels of participation and broader knowledge diffusion. Crossing these two main dimensions – political conditions and technological factors – allows to look at the role of different expert cultures (professional and disciplinary background) and their interaction/intersection with political cultures (e.g. political ecologist, conservationist, environmentalist). These dimensions helps explain different typologies of expert enrollment, whether its participation is more organic to movement areas (expert-activist) or more episodic and linked to single-issue justifications (expert-ally). After a careful analysis of the Italian public debate about high-tech projects, a specific media analysis of the four cases in national and local newspapers, a multivariate ethnographic fieldwork was conducted in both cities that included direct attendance at public meetings, assemblies and demonstrations. Moreover, around 60 in-depth and semi-structured interviews were conducted with public authorities, experts, activists and citizens playing a central role in the mobilization. The outcomes show how conflict, rather than inhibiting it, transforms expertise production into a contentious politics by other means. Being understood as intrinsically linked to political interests, the meaning of contentious expertise needs therefore to be understood in terms of crisis of democratic accountability and legitimation. The use of expertise by social movements has, finally, a clear impact on their structure and composition, giving rise to uncertain and unexpected alliances as well as shifts regarding mechanisms of participation and mobilization.