Violent extremism is a key issue on the regional security agenda in East Africa. However, our review of the relevant literature, complemented by original primary research, suggests that the evidentiary baseline regarding violent extremism in East Africa is modest. The existing literature focuses largely on Somalia and Kenya and serves to underscore that mobilization to extremist violence in the region is diverse. These findings have important implications for development actors seeking to advance “countering violent extremism” (or sometimes “preventing violent extremism”) measures in East Africa. Those measures should be variegated across the states in the region. More generally, development actors seeking to advance countering violent extremism measures in East Africa or elsewhere should ensure that their approaches are evidence-based, responsive to the problems they are designed to address, proportional in light of existing development and security priorities, and effective.