This monograph offers a uniquely comprehensive and in-depth legal account of official secrets in the European Union. It critically analyses their implications for oversight and fundamental rights. Based on forty interviews with practitioners and other stakeholders, it offers an understanding of the practices of official secrets and provides a critical and much-needed perspective on how parliamentary, judicial and administrative oversight institutions deal with access to classified material and the dilemma of oversight to concurrently ensure secrecy necessary for EU security policies and...