Book excerpt: There has been considerable research and authorship on child sexual abuse, however, much of this research has focused on adult perpetrators and child victims. Less attention has been paid to children’s harmful sexual behaviour and the multitude of influences. Harmful Sexual Behaviour in Young Children and Pre-Teens provides evidence-based understanding on: typical sexual development versus harmful sexual behaviour; the prevalence and impacts associated with harmful sexual behaviour; Australian laws, policies and educator responsibilities; responses and support systems for children who display harmful sexual behaviour; and the implications and challenges for future practice. This book provides understandings that directly respond to the recent Australian Royal Commission into Institutional Responses to Child Sexual Abuse recommendation 10.1 to address (a) primary prevention strategies to educate family, community members, carers, and professionals about preventing harmful sexual behaviours (b) secondary prevention strategies to ensure early intervention when harmful sexual behaviours are developing and (c) tertiary intervention strategies to address harmful sexual behaviours.. The authors present a review of psychological, sociological, legal, and educational research to inform and support professionals involved in the wellbeing and education of children to understand, manage, and reduce dysfunctional sexual development in children.