Psychosis is a difficult term to define and is frequently misused not only in the media, but unfortunately among mental health professionals as well. Stigma and fear surround the concept of psychosis, sometimes using the pejorative term “crazy.” This chapter gives a general description of psychotic symptoms and explores the major theories of how all forms of psychosis are linked to the neurotransmitter systems dopamine, serotonin, and glutamate. An overview of specific psychotic disorders, with an emphasis on schizophrenia, is presented here but does not list the diagnostic criteria for all the disorders in which psychosis is either a defining feature or an associated feature. The reader is referred to standard reference sources such as the DSM (Diagnostic and Statistical Manual of the American Psychiatric Association and the ICD (International Classification of Diseases) for that information. Although schizophrenia is emphasized here, we will approach psychosis as a syndrome associated with a variety of disorders that are all targets for the various drugs that treat psychosis and that will be discussed in the following Chapter 5.