Prison standards are an important element of transnational criminal justice. This Article shows how legal standards governing prison conditions emerged at the international and regional levels and considers how, increasingly, they have gained legitimacy. It then describes how these standards are applied in a way that contributes to a recognizable transnational legal order in respect of prison conditions, which has real impact at the national level. The Article pays close attention to the transfer of prisoners between states, as a mechanism that operates transnationally and, in the process, enhances the importance of international prison standards. It concludes that the benefits of common prison standards are mixed. On the positive side, they have the potential to give states that are asked to extradite suspects, or transfer sentenced prisoners, leverage to demand the improvement of prison conditions in the receiving states. There is, however, a risk that states will accept and implicitly endorse sub-standard prison conditions in order to rid themselves of troublesome offenders.