What motivates activists to leave the security of their country and the comforts of home to risk their lives on someone else's behalf? This paper summarizes the current literature on how individuals get involved in transnational social movements, including those that require them to put their lives on the line. I begin by outlining structural explanations for participation and specifically discuss how prior contacts and biographical availability draw new recruits into activism. Then, I discuss cultural explanations, which emphasize meaning-making and collective identities in the recruitment process. Finally, I outline scholarly attempts to bridge structural and cultural explanations as well as offer recommendations for future research.