By using informers to provide intelligence on terrorism, the security and intelligence agencies who handle them gain knowledge of their offences. Charges may then be brought against them, provided evidence supports this course of action. But if imprisoned, an informer no longer has access to the time-sensitive, potentially life-saving intelligence they once had. There is therefore a tension between continuing to use an informer to provide intelligence on terrorism and upholding the law. This tension is at the heart of this book. Terrorist Informers in Northern Ireland analyses prominent terrorist informers such as Agent Stakeknife, and lesser-known examples, who collectively were active throughout Northern Ireland from the 1970s to the present. It looks at both those involved with republican groups and with loyalist groups, and also those working for the police, the armed forces, and MI5. Valuable pieces of the puzzle are unearthed in sources such as court judgments, official reports, and in interviews conducted by the author. The book also analyses the way successive governments, the police, the armed forces, and MI5 have addressed the regulation of terrorist informers' involvement in criminality, as well as allegations of 'collusion' between informers on one hand and the security and intelligence agencies on the other. Accordingly, the book also assesses the varied retrospective investigations into the use of terrorist informers, and therefore the competing needs for secrecy and transparency. As Samantha Newbery's research here shows, although there is a tension between intelligence and the law, this can be successfully navigated.