Non-suicidal self-injury is the intentional destruction of body tissue without suicidal intent and for purposes not socially sanctioned. In this practice-friendly review, the authors summarize the empirical research on who self-injures, why people self-injure, and what treatments have demonstrated effectiveness. Self-injury is more common in adolescents and young adults as compared to adults. Common forms include cutting, severe scratching, burning, and banging or hitting; most individuals who self-injure have used more than one method. Although diagnostically heterogeneous, self-injurers typically exhibit two prominent characteristics: negative emotionality and self-derogation. Self-injury is most often performed to temporarily alleviate intense negative emotions, but may also serve to express self-directed anger or disgust, influence or seek help from others, end periods of dissociation or depersonalization, and help resist suicidal thoughts. Psychotherapies that emphasize emotion regulation, functional assessment, and problem solving appear to be most effective in treating self-injury.