I present a model of the interaction between a government, a terrorist organization, and potential terrorist volunteers in which, as a result of an endogenous choice, individuals with low ability or little education are most likely to volunteer to join the terrorist organization. However, the terrorist organization screens the volunteers for quality. Consequently, the model is consistent with two seemingly contradictory empirical findings. Actual terrorist operatives are not poor or lacking in education. And yet lack of economic opportunity and recessionary economies are positively correlated with terrorism. The model also endogenizes the effect of government counterterrorism on mobilization. Government crackdowns have competing effects on mobilization: they decrease the ability of terrorists to carry out effective attacks (making mobilization less attractive), and they foment ideological opposition to the government and impose negative economic externalities (making mobilization more attractive). This provides conditions under which government crackdowns increase or decrease mobilization.