On 4 July 2014, Abu Bakr al-Baghdadi declared the establishment of an Islamic caliphate, marking the onset of an unprecedented period of state-building by the Islamic State (IS) across Iraq and Syria. At its peak, IS’s state comprised an area of approximately 100,000 square kilometres. The group developed a complex apparatus of governing institutions to manage the lives of the eight million civilians residing in its territory. Collectively, this dissertation contains four papers that focus on under-studied aspects of IS's rebel governance and state-building efforts, offering insights into the civil resistance, the rebel governance, and the broader state-building literature. The first paper challenges prevailing ahistorical narratives of IS's governance by introducing a novel framework, the ‘governance cycle’, to better understand IS's historical and contemporary governance dynamics. The second paper delves into the effectiveness of IS's governance, revealing variations in governance phases at the sub-national level. The third paper explores the categorisation and roles of employees within jihadist groups, demonstrating that IS had two distinct categories of employees who had different roles, benefits, and levels of agency. Finally, the fourth paper analyses the strategic narrative which surrounds IS's state-building project, highlighting its conventional nature, compared to mainstream state-building narratives. These papers are based on extensive primary data collection consisting of 117 interviews with IS members and civilians who lived in IS-controlled territories and an original database of more than 1000 documents, published by IS’s governing institutions.