Two major questions are seldom addressed in the literature on Islamism and opposition social movements more generally: (1) what explains the relative success or failure of Islamist groups in mobilizing a social base and (2) what role do Islamist ideas play in attracting support. Islamist movements vary significantly in their origins, leadership, ideas, and strategies. In answering these important questions, this article offers three main propositions: that under certain conditions, Islamism can emerge as a powerful idea that generates social appeal; that to be successful, Islamist organizations must develop a local Islamist ideology that suits the local social base, rather than tie themselves to a global Islamist agenda,; and that in authoritarian contexts, especially where open mobilization is forbidden, inclusive informal social networks are an essential mechanism for spreading Islamist ideas and protecting group members. Nonetheless, there are limitations to an Islamist movement's ability to grow and bring about political change. The article contributes to an understanding of Islamism and, more broadly, to an understanding of why and how opposition movements emerge and mobilize under authoritarian regimes. The article develops these propositions in a comparative examination of three Islamist groups active in the Central Asian and south Caucasus regions of the former Soviet Union (FSU): Hizb ut-Tahrir al-Islami (HT), the Islamic Renaissance Party of Tajikistan (IRP), and the Islamic Party of Azerbaijan (IPA).