The aim of this thesis was to explore the contribution of attachment theory to the understanding of sexual offending behaviour. The introduction chapter is followed by a systematic literature review (Chapter 2), exploring whether child abusers and rapists differ in attachment style. Chapter 3 investigated the psychometric properties of the Attachment Style Interview (ASI; Bifulco, Moran, Ball, & Bernazzani, 2002). This semi-structured interview demonstrated satisfactory reliability and validity. Chapter 4 consists of an empirical study using a mixed-methods approach to explore the attachment styles of adolescent sex offenders. The Attachment Style Interview for Adolescents (Bifulco, 2012) was used to investigate whether there is a relationship between attachment style and offender status (child abusers, peer abusers). The quantitative results supported the hypotheses that child abusers are more likely to be anxiously attached; whereas peer abusers are more likely to be avoidantly attached. The qualitative results further explored what participants valued in relationships with others, and what represented as barriers for them to make and maintain relationships. The final chapter summarises the findings and implications for practice of this thesis. Overall, this thesis highlights that sex offenders are a heterogeneous group, whose needs are complex and go beyond their sexually harmful behaviours.