This chapter introduces the concept of the private criminal justice system, which comprises every private response to alleged criminal activity. It gives examples of private law enforcement, private criminal settlements, private criminal adjudications, and private dispositions. It argues that the private criminal justice system is the predominant way in which our society responds to crime. Finally, the chapter sets out the goals of the book: (1) to describe the private criminal justice system, which has been understudied in the literature; (2) to explain why it has become so prevalent in society; and (3) to evaluate the social desirability of different aspects of the system.