The literature shows a high prevalence of co-occurring psychiatric disorders (PDs) of any kind in adolescents and adults with intellectual disability (ID) and/or autism spectrum disorder (ASD). The largest epidemiological study on this co-occurrence found a point prevalence of 28.3%, and 40.9% if problem behaviors were included. The rate seems to increase as the neurodevelopmental condition is more severe. Point prevalence estimated in mild ID is 25.4% vs 30.2% in moderate to severe. Similarly, the co-presence of ID and ASD is associated with an overall prevalence of 70%, with around 40% of cases showing two or more psychiatric diagnoses. Attention-deficit/hyperactivity disorder, bipolar disorder, dementia, and schizophrenia are the most frequent PDs in persons with ID. Depression and anxiety disorders are also common although it is disputed whether they are more prevalent than in the general population. Anxiety disorders and attention-deficit/hyperactivity disorder are the most common PDs in children and adults with ASD. Diagnosing PDs in individuals with ID and/or ASD is challenging. In many cases, PDs are not diagnosed due to factors like “diagnostic overshadowing,” atypical onset and presentation of symptomatology, significant cognitive and communication impairments, presence of multi-morbidity, and difficulty in applying assessment tool, diagnostic criteria, and clinical knowledge tailored on the general population. For these reasons, the identification of co-occurring PDs requires trained clinicians and specific assessment tools. Given that mental health issues greatly affect adaptive skills, general functioning, and quality of life of the person with ASD and/or ID, it is crucial to conduct a comprehensive multidimensional assessment and provide accurate diagnoses. This ensures that the most effective interventions can be promptly implemented.