This chapter focuses on secrecy and the so-called closed material procedure (CMP), a mechanism introduced during the late 1990s that enables the State to conceal evidence that is directly relevant to the case from the public and the other party. It utilizes the example of secret evidence to explores and thinks through more systemic challenges to knowledge production and accountability for Extraordinary rendition resulting from the place and role of secrecy and national security within the United Kingdom's constitutional framework. The chapter shows that whilst the Justice and Security Act 2013 was driven primarily by the executive, its adoption and acceptance would not have been possible without some legislative and judicial support. It is concerned about knowledge production and accountability. The chapter analyses that the implicitly based on the assumption that the CMP, and other laws regulating disclosure, was primarily concerned with factual information.