The introduction sets out the central aims of the book: to offer a fresh analysis of the factors that sustained violent conflict and prevented a peace settlement in Northern Ireland for so long and to elaborate the distinctive features of negotiations conducted in secret. It describes the approach taken in the book and argues that violence and negotiation must be analysed together as part of a single process of conflict transformation. It sets out the value of existing work on civil wars, contentious politics, and wartime political orders in analysing negotiation in the case of Northern Ireland. The Northern Ireland conflict provides a revelatory case of back-channel negotiation. It is one of the few conflicts for which there is extensive, reliable primary documentation of clandestine engagement through an intermediary, and this chapter introduces the unique sources on which the book draws. These include private papers, government archives, and interviews with Irish republicans, British and Irish civil servants, the key intermediary, and others.