The title of this presentation is of course an allusion to Howard Becker’s famous text “Whose side are we on?”, in which he addressed the ethical position sociologists should adopt towards the people and groups (especially deviant ones) they study. The issue I want to address in my presentation is a rather distinct one, as the question at stake is not asked by a sociologist to his or her colleagues but asked by the social actors—in that case social movement activists—to the academics who intend to study them, often with a sympathetic (sociological) eye.Many sociologists and political scientists choose to study social movements for which they share some kind of sympathy: they think their causes are legitimate, their grievances are worthy, and their collective struggle needs support. Even if some face difficulties recognizing it within the academy, many hope that their study will help the movement they study to fulfill its aims, sometimes without knowing exactly how but at least that’s what they claim when they negotiate access to the field and ask for interviews. The situation can even be more explicit when the sociologist is him or herself a member of the movement s.he studies, participates to its actions, and publicly shares its aims and grievances. In that case, the observer does not need to negotiate access to the field (as s.he already is part of it), and participant observation is rather a given than a methodological challenge. I’d like to address both situations—being a sympathizer or a member of the studied movement—based on my own experience of two studies I conducted on social movements who I was close to. The first movement was a prostitutes’ mobilization, part of the wider French movement against aids, of which I was a sympathizer and to which I dedicated part of my PhD. The second was a campaign against “double penalty”, meaning the deportation by the French state of delinquents of foreign nationality, that I first joined as an activist then studied as a sociologist. My presentation will first stress the methodological benefits of such situations: being perceived as a movement insider, the sociologist has direct access to some data that an outsider would face difficulties (and need time) to reach. I will especially focus on what, referring to Goffman, I would call backstage activity, during which issues (such as tactics, competition, partnership, manipulations, etc.) are explicitly treated that can’t be dealt with publicly, often because they are considered as breaking with legitimacy constraints. The presentation will also point to the difficulties that result from being a movement insider. As movements are often cleaved groups, the observer risks to be enrolled and enjoined to take side within the militant group, loosing access to members s.he disagrees with. The divergence between what, following Wittgenstein, one could call the scientific vs activist language games can also cause trouble when activists perceive the sociological analysis of their action as a faulted portrait that risks endangering the cause. Being a movement sympathizer or member then exposes to move to another category: that of the traitor.