The role of sexual fantasy should be considered for offenders who have committed sexual offences to assess whether it has contributed to their offending behaviour. In the context of looking at sexual fantasies and sexual offending it may be more helpful to conceptualize deviant sexual fantasy as offence-related fantasy. This chapter first examines the various theories and the research linking offence-related sexual fantasy and sex offending. In their integrated theory of sexual offending, Ward and Beech view the development of deviant sexual fantasy as a product of deficits in managing attachment and mood problems and holding dysfunctional schema. However, there has been inconsistency in the outcome of research exploring the differences between fantasies of sex offenders and non-sex offenders. The chapter concludes by presenting some key points which have important implications in terms of assessment, treatment and management of sex offenders.