Comorbid autism spectrum disorder (ASD) and other mental health conditions are common. However, the recognition and study of this clinical issue is of recent origin. It has recently emerged that certain disorders are more likely to occur with ASD, such as ADHD, depression, anxiety and conduct disorder/challenging behaviors. Developmental factors are significant in that, while ASD presents at a very early age, this is not often the case with the co-occurring disorders noted above. The clinician should be aware of and plan for these potential concerns. Tests that are specifically designed to assess for comorbid mental health issues among persons with ASD are being developed. These methods are recommended given what we know about high rates of comorbidity in this emerging field.