There has been little study of predisposing and precipitating factors of sexually harmful behaviour before adolescence. This thesis seeks to add to the body of knowledge of pre-adolescent onset in male children through quantitative and qualitative examination of the characteristics and circumstances of 27 boys with recorded onset of serious sexually harmful behaviour before the age of ten years referred to the Young Abusers Project, a specialist assessment and treatment service. The client group are children with very high levels of family, psychological and behavioural problems who may not be representative of the wider population of boys with pre-adolescent onset. The study builds upon existing research-informed trauma and attachment based models for adolescent onset. It identifies sexually harmful behaviour as one of many systems shaped by mutually influential external and psychic processes: an important link between attachment and sexual systems is identified. The significance of a sequential pattern of family history of harm to children, hostile/helpless caregiving, progressive neglect and maltreatment in very early childhood on research subjects' resilience to subsequent sexual abuse victimisation is explored. The emergence of sexually harmful behaviour as a self regulating protective adaptation is explained in a three stage model. Predisposing factors in stage one include; genetic potentiality, unresolved parental trauma, early maltreatment, and hostile/helpless caregiving leading to disorganised attachment in infancy and early childhood followed by coercive, punitive-aggressive strategies. Sexual victimisation is seen as a precipitating event in stage two and onset of sexually harmful behaviour identified as an adaptive externalised, defensive mechanism in stage three. The anomaly of apparent premature sexual arousal as a reinforcing agent of preadolescent sexually harmful behaviour considered. Pathways to serious sexual and mental health problems later in life are identified. The study speculates on the emotional impact of working with sexually harmful behaviour on individual professionals and professional systems and recommends changes in practice and training for social work, health, education and criminal justice employees.