Organised by the EPA Section on Child and Adolescent Psychiatry. Diagnosis of autism spectrum disorder (ASD), especially in adolescence and adulthood, can be a challenge even for experienced clinicians. ASD is manifested in a very heterogeneous way, even with the defined diagnostic criteria. Moreover, symptom formation in the same person also changes over time. Moreover, clients with ASD but not diagnosed often turn to psychiatric care with other diagnostic questions, and this often means that interacting and overlapping symptoms make it difficult to recognize the ASD behind surface. Consistent with research, the Diagnostic and Statistical Manual of Mental Disorders allows additional diagnoses among individuals with ASD. The prevalence of comorbid disorders among people with ASD varied across studies: it is suggested that 54-70% of children and adolescents with ASD have an associated psychiatric disorder, 41% of them have two or even more comorbidities and above 18 years even a higher prevalence of psychiatric comorbidity (up to 94%) is described compared to younger people with ASD. Comorbid psychiatric disorders are anxiety, mood-, eating- , obsessive-compulsive - , disruptive -, impulse-control - and conduct disorder, substance use, attention-deficit/hyperactivity,schizophrenia and suicidal behavior. Comorbid psychiatric disorders cause high burden to people with ASD.Therefore, it is insufficient not only to have adequate knowledge of the diagnostic aspects of ASD, but also to have a comprehensive picture about the associated psychiatric disorders and their interaction with ASD. The aim of our course is to help deepen the diagnostic process and aspects of ASD, also in connection with possible comorbid psychiatric disorders.