This chapter provides an overview of what is now generally well-accepted knowledge about nonsuicidal self-injury (NSSI). The chapter lays the foundation for understanding NSSI, addressing the most commonly misunderstood features of NSSI, such as suicidal self-injury and self-destructive behavior and the need to consider multiple methods of NSSI beyond just cutting. The chapter also provides an overview of research on the epidemiology of NSSI, the functions of NSSI, and how NSSI is distinct from other behaviors and disorders, such as suicidal self-injury, self-destructive behavior, and borderline personality disorder.