Many rebel organizations receive significant assistance from external governments, yet the reasons why some rebels attract foreign support while others do not is poorly understood. We analyze factors determining external support for insurgent groups from a principal-agent perspective. We focus on both the supply side, that is, when states are willing to support insurgent groups in other states, and the demand side, that is, when groups are willing to accept such support, with the conditions that this may entail. We test our hypotheses using new disaggregated data on insurgent groups and foreign support. Our results indicate that external rebel support is influenced by characteristics of the rebel group as well as linkages between rebel groups and actors in other countries. More specifically, we find that external support is more likely for moderately strong groups where support is more likely to be offered and accepted, in the presence of transnational constituencies, international rivalries, and when the government receives foreign support.