Although the content of sexual fantasy has been extensively researched, very little contemporary research has investigated the function of sexual fantasy within the context of offending. In this study, a qualitative analysis was used to develop a descriptive model of the phenomena of sexual fantasy during the offence process. Twenty-four adult males convicted of sexual offences provided detailed retrospective descriptions of their thoughts, emotions and behaviours before, during and after their offences. A data-driven approach to model development (grounded theory) was undertaken to analyse the interview transcripts. A preliminary model was developed to elucidate the function of sexual fantasy in the process of sexual offending, as well as the physiological and psychological variables associated with it. The sexual fantasy function model (SFFM) comprises four categories that describe the various functions of sexual fantasy in the offence process. These categories are affect regulation, sexual arousal, coping, and modelling. The strengths of the SFFM are discussed and its clinical implications are reviewed. Finally, the limitations of the study are presented, and future research directions discussed.