One of the major policy concerns surrounding violent conflicts in Afghanistan, Iraq, Libya, Mali, Pakistan, Russia’s North Caucasus region, Somalia, and Syria has been that these struggles may both attract and breed transnational insurgents, or foreign fighters. Yet despite this growing worry, relatively little is known about the ways in which transnational insurgents influence the domestic struggles they join. Existing scholarship assumes that such “outsiders” strengthen domestic opposition movements by bringing with them fighters, weapons, know-how, and access to finanancial resources. Indeed, access to such assets explains why domestic resistance leaders may initially welcome transnational insurgents. Foreign fighters, however, can also weaken domestic insurgencies by introducing new ideas regarding their objectives and how these struggles should be waged. The introduction of new goals and tactics can not only create divisions with opposition movements, but can also complicate the ability of local leaders to attract and maintain vital public support. Domestic resistance leaders’ willingness and ability to adapt the ideas of transnational insurgents to local conditions is key to determining whether and how foreign fighters strengthen homegrown insurgencies.