This introduction presents an overview of the key concepts discussed in this book. The book is a combination of both the analyses which concern criminal informers only as well as analyses dealing with a wider context and thus more general sociology. It seeks to explore what G. Simmel called formal sociology: to find generalities in spite of very different concrete content. The criminological and police science literature has obviously been reviewed as it refers to informers. Historians' and journalists' account of betrayers and their fates after the McCarthy era have also been used. Betrayal not only consists of treachery toward the country but experiences of betrayal are often entangled in relations with family and friends. They will suffer the consequences of the traitor's acts, sometimes in practical ways and sometimes by the experience of having been left out.