Nonsuicidal self-injury (NSSI) is a relatively common behavior engaged in by adolescents. School mental health professionals have identified NSSI as a behavior for which they need more information and practice guidelines. Despite a clear need, few resources exist to help with understanding the development and maintenance of NSSI in adolescents. This article reviews relevant literature from counseling and clinical psychology and related disciplines to clearly outline the theoretical models most relevant to understanding why youth engage in NSSI. The use of case vignettes serves to illustrate relevant theoretical underpinnings and scientific literature. Functional assessment of NSSI is dovetailed with implications for treatment. Suggestions for the practice of school psychology are discussed.