Nonsuicidal self-injury (NSSI) is a major mental health problem in youth worldwide. The World Health Organization has recognized it as among the top five major health threats to adolescents. NSSI is defined as deliberate infliction of direct physical harm to one’s own body without suicidal intent. Recently, NSSI was introduced for the first time in the Diagnostic and Statistical Manual of Mental Disorders 5th Edition (DSM-5), under section III, “Conditions for Further Study.” In most cases, NSSI occurs for the first time around the age of twelve, with great increase in frequency within each year of adolescence. Injuries range from minor cuts to severe injuries, including biting, hitting, and burning oneself. Most self-harming behavior in adolescents ceases in the long-term, but a substantial proportion of adolescents show a continuity of NSSI into young adulthood accompanied by pronounced psychiatric comorbid disorders. A history of NSSI has also been identified as a strong risk factor for suicide attempts in later life. Emotional dysregulation has been acknowledged as the central clinical feature, both in pathogenesis and treatment of NSSI. Existing neurobiological findings increasingly support this clinical model. The development and implementation of effective intervention programs are of the greatest importance for the future.