© 2016 Elsevier Ltd This review will teach readers about non-suicidal self-injury (NSSI), which is a serious and common problem, particularly among adolescents. We shall begin by explaining what we mean by non-suicidal self-injury, and then move on to discuss the epidemiology, causes and outcomes. We shall finish by discussing assessment and treatment, as well as some important research issues. NSSI is meaningfully distinct from suicidal self-injury despite being closely related, and it presents in a variety of forms. Many factors have been implicated in the ontogenesis of NSSI, in particular poor family relationships, early abuse, affective instability and reactivity, impulsivity, psychological illness and distress. While the majority of cases of adolescent NSSI resolve on their own by adulthood, NSSI is associated with a number of adverse physical, psychological, and social outcomes. It is also a common cause of presentation to hospital that will be frequently encountered by paediatricians and emergency department doctors. There is only preliminary, unreplicated, evidence for specific treatments. The most practical approach may be to treat any underlying psychiatric illness, address environmental stressors, and provide a supportive and positive therapeutic environment. Anyone reporting NSSI should be assessed for risk of repetition and suicide, and any physical injuries should be treated.