Non-suicidal self-injury is a common behaviour in adolescents and young adults, and may be associated with mental health disorders, risk of suicidal behaviour (ideation and attempts), and a need for clinical services. Nurses, in particular those working in emergency departments and mental health settings, have a crucial role in the assessment, treatment and care of individuals who have self-injured. It is essential for nurses to assess an individual’s risk of more serious harm or accidental death, regardless of intent. It is also important to understand the variations in non-suicidal self-injurious behaviour in terms of its presentation, features and functions, to provide appropriate person-centred care. Nurses should assist individuals in identifying the triggers or cues for their behaviour, exploring treatment options, and monitoring their behaviour and risk in the long term. This article describes the profile of people who self-injure, and the issues related to assessment and management of such patients presenting in emergency departments. A description of who self-injures and why, and how people self-injure; developmental aspects of these behaviours, including short and long-term outcomes; and the available treatments is presented.