Autism spectrum disorder (ASD) is a particularly important risk factor for challenging behaviours such as aggression, tantrums, self-injury and pica. Adults with ASD have rarely been studied with respect to these problems. This is particularly disconcerting since there are far more adults than children with ASD. In addition, because of adults' increased physical size and longer history of these problems, treating these behaviours effectively is important. Psychological methods, particularly applied behaviour analysis, and pharmacotherapy have been the most frequently addressed treatments for challenging behaviours associated with ASD in the research literature. In many cases, challenging behaviours have clear environmental antecedents. In these cases, behavioural interventions, such as applied behaviour analysis, should be used to reduce the behaviours. When environmental factors cannot be identified or when challenging behaviours are very severe, pharmacological treatments may be necessary in combination with behavioural interventions. Newer antipsychotics are the most researched medications for use with this population. Currently, risperidone and aripiprazole are the only medications that have US FDA approval for the treatment of behaviours associated with ASD, specifically irritability; however, they are indicated for use in children not adults. It is important not to use medications unnecessarily, due to possible side effects associated with their use. Based on available research, some recommendations for the treatment of challenging behaviours of adults (and children) with ASD include the use of functional assessment, side-effect monitoring of medications and behavioural methods whenever possible. Additionally, future research in this area needs to focus more on adults, as most current research has used child samples.