Women have served in many capacities in ethnonationalist armed groups during wartime, including the role of combatant. However, most theories that discuss the recruitment of armed fighters often assume that combatants are male or that the same recruitment logic that applies to men also applies to women. In this dissertation, I challenge this assumption and provide answers to two questions surrounding the recruitment of female combatants. The first question is, what factors affect ethnic armed groups' decision to recruit women? And the second is, at what point during the conflict do armed ethnic groups make the decision to recruit female fighters? To answer these questions I create a dynamic theory of the recruitment of female combatants. Drawing on the recruitment literature and scholarship on war and gender, I identify six variables that affect ethnic armed group's decision to recruit female fighters, these are: type of resource endowment, conflict intensity, participatory governance system, group fragmentation, gender inclusive policies and politically autonomous women's organization. I argue that contingent on the conflict phase---guerrilla activity phase or civil war phase---in which recruitment occurs, these variables either increase or decrease the likelihood that women are recruited to serve in combat positions. To test my theory, I conduct qualitative analyses of three different conflicts involving the following ethnic armed groups---the Karen National Union/Liberation Army, the Liberation Tigers of Tamil Eelam, the Eritrean Liberation Front and the Eritrean People's Liberation Front. Drawing on case study evidence, archival research and fieldwork in Thailand and Burma/Myanmar, I found support for a number of hypotheses advanced in this dissertation. For example, I found that type of resource endowment, conflict intensity and gender inclusive policies are the best indicators that an ethnic armed group will recruit women. Additionally, I found that armed ethnonationalist organizations are more likely to recruit women fighters in the civil war phase in comparison to the guerrilla activity phase. These findings help expand our knowledge in the research areas of insurgent recruitment and mobilization, women's participation in conflict and conflict dynamics.