Islamism in Indonesia and Malaysia has undergone a fascinating transformation from social movement roots to mainstream politics. How did this take place, and to what ends? Drawing on social movement theories, this Element explains this transformation by focusing on key Islamic social movements in these two countries. It argues: first, that the popularity and appeal of Islamism in Indonesia and Malaysia cannot be understood without appreciating how these social movements have enabled and facilitated mobilization; and second, that it is precisely these roots in civil societal mobilization that account for the enduring influence of Islamist politics evident in how Islamic social movements have shaped and transformed the political landscape. These arguments will be developed by unpacking how Islamist ideas took root in social movement settings, the kinds of institutional and organizational structures through which these ideas were advanced, and the changing political landscape that facilitated these processes.