Suicide is one of the major causes of death in young people, in whom suicide can occur in clusters. In this Review, we have investigated definitions and epidemiology of such clusters, the factors associated with them, mechanisms by which they occur, and means of intervening and preventing them. Clustering of suicidal behaviour is more common in young people (<25 years) than adults. Suicide clusters can occur as a greater number of episodes than expected at a specific location, including in institutions (eg, schools, universities, psychiatric units, and youth offender units).