The central focus of this chapter will be to describe the theory and practice of critical friendship in teacher professional development, paying special attention to the ways in which participants in small professional learning communities (PLCs) create spaces in which to experience discomfort together for the purpose of sustaining their own transformation as practitioners. Using protocols (prescribed turn-taking mechanisms) as social processes to negotiate and then arrive at explicitly named norms and agreements, PLCs that use critical friendship as their goal aim to create the conditions for personal and communal transformation of both their members and their institutional contexts.