This paper differentiates between sexual offending behaviour and challenging behaviour in people with an intellectual disability. There is growing awareness that people with an intellectual disability can perform both classes of behaviour. The definitions of sexual offending and challenging behaviour are almost interchangeable, due to the poor construction of the term challenging behaviour. It is when clinicians attempt to view sexual offending from within the framework and underpinning philosophy of the challenging behaviour model that the magnitude of the mismatch emerges. In this paper, the basic definitions are elaborated on and the different models of assessment discussed, as is the approach taken towards the treatment of the two kinds of behaviour. The corresponding notions of intent, criminal intent and communicative intent are also compared. The implications of interpreting sex offending behaviour as challenging behaviour are highlighted. In the conclusion of this discussion functional analysis is put forward as a technique that may aid in the assessment of sex offending.