More than 45 years after its adoption, the State Immunity Act 1978 remains the principal source of rules as to the immunities and privileges of foreign States and related persons before the courts of the United Kingdom. Although the legislative text has changed relatively little in that period, the ever-growing body of case law in the UK courts has more clearly defined its contours and limitations and clarified its relationship with other rules of municipal and international law. This work provides a detailed commentary on the provisions and operation of the State Immunity Act 1978 in the United Kingdom as well as the surrounding legal framework. Delivering a comprehensive section-by-section analysis, it outlines the context, background, and legislative history to the Act. Furthermore, it explores the Act's relationship to other aspects of domestic law and to international law, including common law, the European Convention on State Immunity, customary international law, human rights instruments, and other international law immunities and conflict of laws rules. In examining this remarkable and influential piece of legislation, the work highlights issues that remain controversial and proposes solutions to the doctrinal and practical difficulties that arise when applying its provisions. Written by authors with significant experience in dealing with issues of immunity as practitioners as well as extensive knowledge of the surrounding academic debates, this commentary provides expert guidance for practitioners and judges faced with questions of state immunity law in the UK and internationally, and will be of great value to academics with an interest in this branch of international law.