Background: The present study examines understanding of challenging behaviour among a sample of children and adolescents with a moderate intellectual disability, and investigates their behavioural intentions towards peers with challenging behaviour. Methods: The study involved the collection of quantitative and qualitative data. In the quantitative part of the study participants (n = 39) completed a modified Friendship Activity Scale (FAS) following the presentation of vignettes depicting individuals with challenging and non-challenging behaviour. In the qualitative part of the study, participants (n = 31) took part in a semi-structured interview that sought their views on the causes of the challenging behaviour described in one of the vignettes. Results: Analysis of the data from the FAS indicates that participants have significantly more positive intentions towards a vignette character that does not engage in challenging behaviour. Content analysis of the qualitative data indicates that participants hold a variety of beliefs about the causes of challenging behaviour. Suggestions include the possibility that it is a response to transient emotional states and to external events. Conclusions: The findings are consistent with the findings of other studies on young people's understanding of and attitudes towards peers with atypical behaviour. The theoretical and clinical implications for young people with intellectual disabilities are discussed.