There is growing recognition that individuals with autism spectrum disorder (ASD) struggle with anxiety, depression and anger: symptoms that are not part of the diagnostic criteria for the disorder. Psychiatric comorbidity is common, although prevalence estimates vary widely. Rates vary from 28 to 84% of individuals with ASD meeting criteria for at least one other psychiatric disorder. Results of a meta-analysis indicate that the pooled prevalence estimate of comorbidity is 68%. Individuals with ASD have more emotional and behavioural problems than the general population and other clinical groups. It is important to understand this comorbidity because of its high prevalence, impact on both current functioning and long-term outcomes, persistence of symptoms, and cost to the individual, their family and society. The recognition and assessment of emotional and behavioural problems in individuals with ASD is challenging. Comorbid psychiatric disorders should be assessed and diagnosed early to support the individual and their family better in school/work and home environments in order to improve the quality of their lives and improve longer-term outcomes.