Schizophrenia is the most common and important type of a primary psychotic disorder (i.e., not caused by drugs or a medical condition), representing more than half of all patients with a psychotic disorder. In this chapter, we will follow the DSM-V framework and review schizophrenia and related disorders as schizophrenia spectrum disorders (i.e., schizoaffective disorder, delusional disorder, schizophreniform disorder, brief psychotic disorder, and shared psychotic disorder). Schizotypal personality disorder, which is closely related (and often considered a schizophrenia spectrum disorder), is discussed elsewhere in this book.