NSSI is discussed as a public health issue. Studies from Western countries show that one in five young adults has, at some point in their life, deliberately self-injured without suicidal intent. The high prevalence means that NSSI can no longer be explained exclusively as an individual phenomenon springing from the person’s personal background. Research into the causes of NSSI must include cultural and sociological aspects of modern life. Prevalence varies across age groups and is reviewed here for children, adolescents, adults and elderly people. Gender differences are also discussed. Most studies find that more females engage in NSSI and that the preferred methods of NSSI differs between the genders, with a tendency for more females to cut themselves and more men to hit themselves. The gender differences are explained in part by the fact that previous studies have excluded men who self-harmed, which led to NSSI being perceived as a largely feminine disorder. This in turn caused doctors to look more for self-harm in female patients – and finding what they were looking for, namely signs of cutting. NSSI among men who self-injure by hitting themselves has therefore been overlooked in the healthcare sector.