This research investigates an often forgotten aspect of collective action, its imaginary dimension, by focusing on an overlooked concept: utopia. According to me, utopia is both a form of discourse and a set of particular practices. Thus, to be called utopian, a discourse has to include, first, a rejection of the existing society, and second, if not a clear conception of what another world might look like, at least the idea that another society is possible and desirable. And, to be called utopian, practices need to be an attempt to create here and now at least some of the features of this utopian discourse, in the hope of a spread in the rest of society. This definition of utopia has been inscribed in a theoretical reflection linking the concept to the other explanatory dimensions of social movements in order to see how bringing utopia in the debate can highlight some neglected aspects of collective action. In order to investigate the presence and the impact of utopia, I have decided to conduct a comparative case study within a single social movement: political consumerism. I understand political consumerism as a social movement in which a network of individual and collective actors criticize and try to differentiate themselves from traditional consumerism by politicizing the act of buying in order to search for and promote other types of consumption. Thus I have observed four groups from two countries, France and the UK: two convivia of the Slow Food organization, a group of de-growth promoters surrounding the Casseurs de Pub and an intentional community living with the principles of ecovillages called Redfield Community. Through the study of these groups, I have been able to uncover the “grammatical structure” of their utopias and, then, the content of these utopias. Once this done, I have observed the interactions between utopian discourses and utopian practices. Individually, utopias, through these interactions, end up shaping the whole life of the activists and, collectively, they have consequences for many of the choices groups make. They constrain them so that they choose means that are consistent with their ends, but they also help them move beyond some of the difficulties the groups meet. These interactions also involve an emotional work that is both directed towards the activists themselves and towards outsiders. Finally, I have looked at the dialectical relationship that exists between utopia and the involvement people have in collective action. Activists use their utopias more or less consciously to have an impact on society, convince others of the rightness of their cause and protect themselves against attacks from others. Moreover, utopia has a role in the positioning of these groups in the spaces of social movements and politics.