Why was Anti-Slavery International (ASI) so effective at changing norms slavery and even mobilizing the support that ended the transatlantic slave trade at the end of the nineteenth century, and why has that success not continued on into subsequent eras? This article claims that ASI's organizational structure is the key to understanding why its accomplishments in earlier eras have yet to be replicated, and why today it struggles to make modern forms of slavery, such as human trafficking, salient political issues. (...) Organizational structure is defined by how an NGO distributes power over agenda-setting (proposal and enforcement power) and its implementation. Those NGOs that centralize agenda-setting and decentralize the implementation of that agenda will be most effective at changing international norms. This paper demonstrates the tractability of that claim with a comparative analysis of ASI past and present to show that changes in organizational structure have led to differences in their effect on international norms, in spite of the fact that slavery in its modern forms persists as a political and social problem. (shrink)