This paper applies the concept of “communities of practice” to al-Muhajiroun (“the Emigrants”), an outlawed activist network that seeks to create an Islamic caliphate in Britain and the West through activism and proselytizing. Responding to recent studies on terrorism learning and adaptation, the author argues that focusing exclusively on the outputs of learning is unsatisfactory. Instead scholars should analyze learning as a process and unpack the causal mechanisms behind it. To support his within-case analysis, the author draws on extensive field work, including interviews and ethnographic observation. Newcomers to al-Muhajiroun learn the community’s norms and practices through repeated interactions with more experienced activists. These interactions take place in study circles and through companionship. Activists also learn by doing, preaching the Emigrants’ Salafi-Islamist ideology at da’wah stalls and protesting against the West’s “war on Islam” at demonstrations. The more they do, the better they become at performing the network’s high-risk activism, and the more deeply committed they become to its community of practice. However, far from allowing activists to adapt seamlessly to all challenges, the Emigrants’ insular and dogmatic community of practice creates its own problems, hindering its ability to innovate, expand, and thrive in an increasingly hostile environment.