The lecture traces public international law’s response to the international slave trade and modern slavery from the early seventeenth century to the present day. It will be shown that each step of the formal outlawing of these practices was met by new forms of forced labour. It will be argued that collective enforcement was an effective approach to suppress the slave trade. However, the same cannot be said about the creation of state obligations to suppress slavery and forced labour or for a human rights-based approach to these practices.