Histories of sex crime generally ignore boys. We know very little about how individuals and institutions classified particular boys and particular behaviours, and what ideas informed this practice. Structured around the concept of knowledge, this book untangles complex and contradictory ideas around age, class, sexuality and ethnicity, to show how these discourses shaped the prosecution and protection of boys in England and Australia between 1870 and 1930. Taking readers into the industrial heart of West Yorkshire and the wilds of the Queensland frontier, and into the courtrooms, boys' clubs, reformatories, and schools and factories of the late nineteenth and early twentieth centuries, this book asks what ordinary people, professionals and the state knew about boys, sex and crime, and what they did about it. More than an account of judicial policy and practice, it provides a rich social history of boys, sexuality and crime to think about who and what has invoked, or avoided, the attention of the criminal law, at what times, and for what reasons. This book makes a contribution to compelling contemporary debates about child sexual abuse, exploring a timely question of interest to historians, legal theorists, and professionals. It historicises current discussions around youth sexuality, and sexual abuse, questions preconceptions and challenges stereotypes, and introduces new themes into the historiography.