This chapter uses the United Kingdom’s participation in the Indian Ocean Tuna Commission (IOTC) on behalf of the Chagos Archipelago, which is contested by Mauritius, as a case study to examine the issue of regional fisheries management organisations or arrangements (RFMOs) participation by former colonial powers on behalf of overseas territories subject to decolonisation claims by other RFMO members. The chapter first provides an overview of the international legal framework concerning participation in RFMOs. Next, it examines the legal status and pending decolonisation of the Chagos Archipelago. The remainder of this chapter analyses Mauritius’ challenge to the United Kingdom’s IOTC membership on behalf of the Chagos Archipelago within the IOTC’s legal and institutional framework.