A substantial body of empirical research shows that challenging behaviours, such as aggression and self-injury, are significantly influenced by the broad and immediate social and material environment. Causal models typically draw on operant learning theory to account for this association with implications for assessment and intervention. As this explanation has the capacity to predict most incidents of challenging behaviour at cohort and individual levels and, consequently, successful intervention, it has increasingly been accepted as a complete and sufficient account of the determinants of challenging behaviour. However, there is substantial evidence that person characteristics are associated with challenging behaviour and this evidence extends from broad pervasive characteristics, such as degree of intellectual disability and the presence of autism spectrum disorder, to transient influences, such as the interaction between a specific type of environmental event and a specific motivational state associated with a genetic disorder. These characteristics warrant examination, explanation and assessment in order to: 1) provide a more complete account of challenging behaviour that incorporates all robust empirical findings in the research literature, 2) acknowledge that the lived experience of people with an intellectual disability may differ in ways that make challenging behaviour more understandable and 3) increase the effectiveness of interventions. In this presentation relevant person characteristics are described and a model of challenging behaviour that acknowledges both person and environmental characteristics is outlined.