This issue of Journal of Clinical Psychology: In Session is devoted to the assessment and treatment of non-suicidal self-injury (henceforth self-injury). In this introductory article, the author describes the scope of behaviors referenced by the term self-injury, establishes the historical and current relevance of self-injury for clinical practice, and summarizes the articles that appear in this issue. Self-injury is the intentional destruction of body tissue without suicidal intent and for purposes not socially sanctioned. This issue addresses many aspects of self-injury, including its prevalence, functions, assessment, and treatment. The articles feature case illustrations and treatment recommendations, and all have direct implications for clinical practice.