This book contends that modern concerns surrounding the UK State\u2019s investigation of communications (and, more recently, data), whether at rest or in transit, are in fact nothing new. It evidences how, whether using common law, the Royal Prerogative, or statutes to provide a lawful basis for a state practice traceable to at least 1324, the underlying policy rationale has always been that first publicly articulated in Cromwell\u2019s initial Postage Act 1657, namely the protection of British \u2018national security\u2019, broadly construed. It further illustrates how developments in communications technology led to Executive assumptions of relevant investigatory powers, administered in conditions of relative secrecy. In demonstrating the key role played throughout history by communications service providers, the book also charts how the evolution of the UK Intelligence Community, entry into the \u2018UKUSA\u2019 communications intelligence-sharing agreement 1946, and intelligence .