This chapter describes the phenomenon of psychosis and how it presents in patients with PWS. Characteristics of psychosis, including delusions, hallucinations, disorganized thinking, abnormal motor behavior, or negative symptoms are described. Although the classical presentation of schizophrenia, schizophreniform, and other schizoid disorders is uncommon in PWS, there are other less-recognized psychotic presentations that are explained in the text. Delirium, despite not being considered a psychotic disorder, is important to recognize due to its association with medications patients with PWS are prescribed. Cycloid psychosis is another phenomenon that, although not formally recognized in the DSM-5, is frequently seen in PWS. The sudden onset and cycling between normalcy and psychosis makes it difficult to diagnose for providers who are unfamiliar with it. We emphasize the need for immediate medical attention and treatment in the case of suspected psychosis. Some commonly used strategies for management are discussed. Continued monitoring by mental healthcare providers and need for maintenance treatment is discussed.