In this paper, we analyze transnational support for insurgent groups from principal agent perspective. We focus on both the supply side, i.e., when other states are willing to support insurgent groups in other states, and the demand side, i.e., when groups are willing to accept such support, with the strings that this may entail. We test our hypotheses using new disaggregated data on insurgent groups. Our results suggest that transnational rebel support is influenced both by characteristics of the rebel groups as well as linkages between rebel groups and actors in other countries. More specifically, we find that transnational support is more likely for moderately strong groups, where support is more likely to be influential, in the presence of transnational constituencies, and when the government receives foreign support. By contrast, conventional country characteristics believed to influence the outbreak of civil war appear to be unrelated to whether rebels receive transnational support.