Harmful sexual behaviour in children is a challenging concept to consider; it is fraught with difficulties in definition and acknowledging such behaviour as harmful in order to pass the threshold of criminal behaviour. Once determined as harmful, the additional challenge is then understanding areas of concern and risk. This chapter will consider the prevalence and definition of harmful sexual behaviour in male adolescents and the developmental factors that underpin this. This is followed by considering the important components of risk and intervention, such as effective formulation of the behaviour, risk scenario planning and protective factors. The chapter will then conclude with a brief case study to illustrate how this may present in practice. The literature is less developed in regard to females who engage in harmful sexual behaviour, and even less so in younger children. As such, this chapter will predominantly focus on the child as the adolescent male, although it would be expected for some degree of extrapolation to younger children and females.