This research note considers the role of Parliament in relation to the cession of the Chagos Islands to Mauritius. A treaty ceding sovereign British territory to a foreign power cannot be ratified unless Parliament enacts legislation enabling ratification. In considering whether to enact such legislation, Parliament will have an opportunity to reject ratification of the Treaty – or to impose conditions on ratification. In deliberating about ratification, Parliament should question the Government’s rationale for the Treaty, hear from the Chagossians, consider the risks to the environment, and evaluate the new legal and security risks that this treaty poses to United Kingdom and thus also Anglo-American security interests. The note argues that Parliament should refuse to support ratification of this treaty, instead maintaining the status quo. In the alternative, Parliament should make ratification contingent on certain conditions having been met and/or further assurances having been secured – and should also legislate to give effect in our domestic law to the UK’s understanding of the treaty.