This chapter presents a study carried out in two CoPs (mainly made up of teachers) that were set-up in the same French secondary school; each of these was to implement a project that was considered innovative. The study is based on a theoretical framework aimed at articulating the notion of CoPs within a socio-cognitive approach to the professional development of teachers. During the research, the use of an ethnographical approach allowed for the combination of both a researcher stance and a participant stance without confusing the two. The analysis of the results yielded a better understanding of the content of the project, teaching practices (especially in their collective form) and the professional knowledge constructed and mobilized within a CoP. These results are used as a basis to uncover various elements that can be transferred to other contexts with regard to understanding how CoPs are set-up and operate. CoPs appear to operate in a mode that is comparatively subversive to the organization of teachers' work and state schools as they act as an endorsement of more autonomy for actors and organizations.