This chapter covers the initial evaluation and management of a psychotic emergency department patient with particular emphasis on the process of separating psychiatric causes from medical causes of psychosis. Hallucinations, delusions, thought disorganization, agitation, and catatonia are the most common features of psychosis. Emergency physicians have a primary responsibility to determine which category, organic or functional, defines a patient's psychotic episode. Substance-induced toxicity is a more common cause of acute delirium in children, and should be considered early in the evaluation. Dementia (particularly vascular dementia and Alzheimer's disease) predisposes patients to psychosis that may require inpatient psychiatric management. The etiology of postpartum psychosis is unknown but familial susceptibility suggests a genetic link, and rapid hormone changes seem to play a triggering role. The chapter discusses the management of agitation, and provides information on the medical screening examination, which allows for ultimate categorization of psychosis and appropriate disposition.