This text provides a clear and accessible introduction to the Investigatory Powers Act, a foundational piece of UK national security law. This act repeals part I, chapters 1 and 2 of the Regulation of Investigatory Powers Act 2000 and other surveillance legislation. This new legislation is the blueprint for how state agencies, the police, and internet and telephone companies protect privacy and extract data and information to protect the public from terrorism and is used to prosecute serious criminals. This legislation implements some parts of the recent comprehensive report by David Anderson QC, A Question of Trust: Report of the Investigatory Powers Review, it will put the Data Retention and Investigatory Powers Act 2014 on a substantive footing and attempt to bring existing legislation up to date to reflect technological advances. The passage of the bill was highly controversial and subject to considerable public criticism by the media and civil liberties and human rights groups as well as lobbying by insiders from the Security Service, the Secret Intelligence Service, GCHQ and the police. The Joint Committee on the Draft Investigatory Powers Bill criticised the Bill's definition of 'data' as unclear, unhelpful, and recursive and expressed concerns about the meaning of the term 'Internet Connection Record' as well as the Home Secretary's failure to make sufficient case as to the feasibility of their collection, retention, and use by law enforcement. All of these issues indicate the questions that practitioners will have to face when In the absence of other sources of reliable interpretation for practitioners, this Blackstone's Guide is essential in helping readers navigate and understand the new and complex set of provisions.