This chapter reviews the acute treatment process from evaluation and determination of the disease, which may or may not have a psychiatric origin, to stabilization. Psychosis is disruption in perception, organization of speech and/or organization of behavior. There are several disorders related to psychosis: brief psychotic disorder, schizophreniform, schizophrenia, severe mood disorders (depression or mania) with psychosis, schizoaffective disorder, delusional disorder, and shared psychotic disorder. Stabilization of the psychiatric patient in the emergency department (ED) depends largely on the presenting symptoms but can be thought of as having three main components: de-escalation, treatment, and evaluation of safety. De-escalation is needed for the agitated patient, to ensure safety. There are various treatment strategies for psychosis; the decision is based on several factors, such as patient preference, cost, and access to care. Disposition is largely determined on severity of illness.