This chapter delivers an account of the way in which the engagement between States and the United Nations human rights treaty bodies plays a clear, but often overlooked, role in shaping customary international law. It proceeds from the accepted notion that international organisations contribute to international law-making in a number of ways. It argues that the responses by States to human rights treaty body interpretations support a conclusion that treaty bodies can, and do, contribute to the development of customary international law through their relationships with States parties. As a starting point, this chapter delivers an account of the treaty bodies as primary interpreters of human rights treaties and contributors to the development of human rights law followed by consideration of the prohibition against torture as a human right that is also recognised as a customary rule of international law. While it is clear that the core prohibition against torture is undoubtedly recognised in customary international law, the analysis demonstrates that further dimensions of the prohibition reflecting treaty body interpretations are on the horizon.