Understanding symptoms is crucial for both diagnostic assessment and treatment. Using conventional criteria to assess psychosis in people with intellectual disabilities can be challenging due to communication difficulties and because language deterioration is a hallmark of psychosis. In general, the same symptoms may appear in various mental health conditions. For people with limited ability to report their inner states or symptoms, observing behavioural equivalents may be a solution. The risk of misattributing symptoms, such as interpreting idiosyncratic beliefs and behaviours in people with intellectual disabilities as delusions, is addressed in the chapter on differential diagnostics.