Recent decades have seen an explosion of transnational networking and activism, but participation varies widely around the globe. Using negative binomial regression, we explore how national and global political and economic factors shape this "uneven geography" of participation in transnational social movement organizations (TSMOs). Contrary to assumptions in popular discourse, we find a continued importance of the state and limited importance of global economic integration in determining participation in transnational associations. But while ties to the global economy do not significantly impact participation, a country's links to global institutions enhance opportunities for transnational activism. Rich countries' citizens are more active transnationally, but low-income countries with strong ties to the global polity are also more tied to global activist networks. This suggests that TSMOs do not simply reproduce world-system stratification, but--aided by a supportive institutional environment--they help sow the seeds for its transformation.