The members of the French Green associations, like those of European and North American movements, are well-educated and belong to the upper and middle classes. They can be defined as an “epistemic community”, using their intellectual skills as a resource, exploiting scientific discourse and evidence and undertaking sophisticated forms of collective action which exploit science, the law and the media. When we consider the trajectories of Green activists, through a study of the ways in which they represent themselves, a common theme emerges : the weight accorded to “experts” in militant associations. However, an analysis at the microsocial level of the background, trajectory and activities (political, associational or religious) of these activists reveals differences which make the appearance of the “expert-activist” a more complex phenomenon than might appear at first. Three distinct types of trajectory – political, scientific and reactive – can help to account for the internal disputes which mark the Green movement. Another factor is the gap which separates two generations of activists : we identify two types of activism linked to two historical stages – the activism of the 1970s, and the increasing professionalism of the 1990s. Where the “Founding Fathers” of the Green movement in France developed their own forms of expertise, the militants of the new generation must have a proper academic background if they are to become Green experts-activists. External constraints have had a powerful effect in turning simple activism into professional expertise.