This paper analyzes the organizational and ideological evolution of ISIS sleeper cells to illustrate how ideology sustains terrorist structures. Islamist terrorist groups such as ISIS employ diverse frameworks—from hierarchical chains to flexible network models. These include elements such as recruitment units, financing structures, and covert cells. Despite their different forms, these structures rely on a “unity of thought” as the ideological glue. The research explores how sleeper cells adapted following the collapse of ISIS’s territorial base. It examines how ideological fractures emerged, how cells reorganized, and how recruitment and radicalization persisted across regions—from Africa’s Sahel to Europe and Moscow. The study emphasizes the transformation from centralized authority to decentralized, hidden operational models. The study applies a descriptive method, using primary sources, interviews with former ISIS members, documents, and previous studies. It is structured around two main pillars: (1) the organizational structures of ISIS sleeper cells, including their formation and operational methods; and (2) the ideological dimension, including the shift in doctrine, the emergence of new factions, and the reconfiguration of propaganda strategies. Ultimately, this paper seeks to provide insight into the adaptive strategies of ISIS and the long-term threat posed by ideologically driven clandestine structures.