Autism Spectrum Disorder (ASD) is associated with higher rates of comorbid mental disorder, especially at times of increased stress and change. There are various challenges for mental health professionals working with this group - the recognition and diagnosis of ASD where it has not been previously diagnosed; differentiating between ASD and symptoms of a comorbid mental disorder; and providing an intervention that takes into account both the ASD and psychiatric comorbidity.This manual has been developed by Orygen Youth Health Clinical Program, with support from the Autism State Plan (Victorian Government 2009) to assist mental health professionals working with adolescents and young adults with Autism Spectrum Disorder and comorbid mental disorders. The manual provides tools to assist with the recognition, formulation and treatment of ASD in specialist health settings. It includes some developmentally appropriate resources and visual aids to support clinical work.