While secessionist movements are often understood and portrayed as unitary actors, the majority of these movements consists of multiple factions and organizations that are embroiled in competition or even violent conflict with one another. Cohesion in secessionist movements is thus the exception rather than the rule. What characterizes those movements that do manage to achieve cohesion? And how is it sustained? This exploratory thesis seeks to improve our understanding of cohesion by developing an original typology of cohesion, as well a novel theoretical framework to explain its emergence. With regard to the former, this thesis argues that movement cohesion can take three different forms: (1) unity, in which the movement consists of only one organization that is not divided into factions; (2) synergy, in which different organizations cooperate and coordinate their actions; and (3) hegemony, in which the concentration of power in a single organization allows it to dominate all others. To explain the achievement of these different types of cohesion, the theoretical framework presented in this thesis is structured around four sets of interactions. Firstly, interactions between the movement and the state it confronts are expected to increase the likelihood of cohesion if they generate perceptions of existential threat. Secondly, interactions between the movement and its external sponsors are expected to foster cohesion if external support is either conditional on cooperation, or selectively provided to a single organization. Third, interactions between the movement and the civilian population it represents can also facilitate the achievement of cohesion, either if civilians make their support conditional on cooperation or if a secessionist organization develops governance structures through which it can monopolize popular support. And fourth, interactions within the movement itself, particularly the strategic use of internal violence, can also contribute to cohesion. This theoretical framework is applied to the cases of Western Sahara, Nagorno-Karabakh and Eritrea, demonstrating that there are different pathways to (different forms of) secessionist movement cohesion.