Rhetoric is a way to explain policy problem framing by recognising the practical necessity to persuade audiences in contextual situations. Modern slavery and human trafficking is a complex and emotive problem, simplified through rhetorical demands to motivate an audience of supporters. This article analyses rhetoric by 212 UK anti-trafficking and -slavery non-government organisations (NGOs) to uncover rhetorical practices and their effects on policy framing, supplemented by archival research to compare past and present anti-slavery oratory. Our data show NGOs use rhetoric to motivate supporters and promote a humanitarian problem frame, in opposition to a state-driven security frame. Findings confirm other research in identifying an emphasis on female victims and on sexual over labour exploitation. Past and present rhetoric are equivalent in terms of liberal, Christian values (ethos) and appeals to pathos through sympathy for victims. Historical rhetoric is distinctive in arguing for the equal human status of slaves, whereas contemporary activists argue victims are denied agency. Contemporary rhetoric represses the question of migration, whereas past rhetoric is more deliberative. Rhetoric varies with the requirements of persuasion related to contextual distance, between unlike humans in the past, but in regard to geographical distance today.