This detailed, practitioner text, explains national security law in all its aspects. It collates and explains the core elements of the law, both substantive and procedural, and the practical issues which may arise in national security litigation. The book draws on the professional experience of a team of expert contributors. The first part explores the meaning of "national security", examines the respective roles in this area of Parliament, the executive and the courts, and explains the law relating to the security and intelligence agencies, their powers and oversight. The core of the book addresses the various executive measures used to disrupt terrorism, espionage and other hostile state activity, usually on the basis of secret intelligence, and the civil proceedings that may result from executive action taken for the national security purposes. The third part addresses national security and the criminal law. The remaining chapters address national security law in such diverse contexts as inquests, inquiries, employment, vetting, family, freedom of information, and data protection proceedings. National security law is now of relevance to a wide range of practising lawyers, judges, legislators, policymakers, oversight bodies, and academic experts working in a variety of legal fields well beyond public law. The highly-specialised nature of the topic make this book a vital text not only for those seeking an overview of the law, but also for experienced practitioners instructed to act in proceedings in which national security issues may arise. The intense media and public scrutiny which accompanies many national security cases will also make this book of interest to a wider audience seeking to understand the legal context of such cases.