The differential diagnostic assessment, including the exclusion of other explanations for the symptoms, encompasses a number of factors that may complicate the diagnostic assessment of psychosis in people with intellectual disabilities. Several conditions may be mistaken for psychosis in people with intellectual disabilities. In this chapter, the following areas are addressed as possible differential diagnostic categories to psychosis: autism spectrum disorder, obsessive–compulsive disorder (OCD), post-traumatic stress disorder (PTSD), neurological disorders, and physical disorders causing pain.