Why do some rebel groups win while others lose? Current explanations for why the outcomes of insurgencies vary tend to either overlook the role that the population plays in supporting an insurgency or fail to specify how the requirements of a rebel group's mobilization of popular support might have their own effect on conflict outcome. In this dissertation, I develop a theory of conflict outcome that links a rebel group's mobilization of popular support to organizational and administrative reforms, which, in turn, affect an insurgency's chances of winning or losing. I argue that all rebel groups face a core "Insurgent's Dilemma." On the one hand, insurgent groups often need to become organizationally centralized and have a large-scale administrative presence in order to mobilize a large amount of popular support, because with higher levels of popular support comes a greater chance of defeating a regime. Yet the same organizational and administrative characteristics that allow for effective mobilization actually favour the state in its efforts to destroy an insurgency. On the other hand, an insurgency can abjure centralization and a large-scale presence and avoid adopting characteristics that favour the state's counterinsurgency efforts. Doing so, however, limits the amount of popular support that an insurgency can ultimately mobilize, which effectively means that the group is unlikely to succeed, even if it is hard to defeat. Throughout the development of the mobilization and conflict outcome theory, I substantiate each point with qualitative historical evidence from a number of insurgencies. I also explicitly test three hypotheses that are derived from the theory using descriptive statistics from 21 insurgencies. Finally, I test the proposed mechanisms against four cases (the Mau Mau in Kenya, the Shining Path in Peru, the Mujahideen in Afghanistan, and the EPLF in Eritrea).