Over the past thirty years or so the study of American fraternity has been used to explore a variety of phenomena in the nation's evolution, especially around the turn of the twentieth century. Fraternities have been used to understand the exploration, taming and exploitation of the West. They have been shown to represent proof of the various turn-of-the century crises of gender, race and ethnicity. They have been seen as the very embodiment of bygone caring, sharing, communities. However, among the aspects to have escaped attention is the importance of fraternity in criminal organizations. Given that crime, then as now, was seen as one of the most pressing of social issues, and given that over these years there was a deep suspicion that there were a variety of ultra-secret fraternities organizing, facilitating and manipulating wide-ranging criminal activities, this may be considered a little odd. This article investigates the idea that there was really such a thing as a genuine criminal “fraternity.” Looking at three of the most famous of such organizations – the Ku Klux Klan (KKK), the Molly Maguires and the Mafia – it demonstrates that not only were ideas of fraternity central to their very existence, but they are also crucial to our understanding both of them and of the period in which they were situated.