This comprehensive chapter examines the nature of the psychosis spectrum from multiple perspectives. Beginning with a definition of psychosis and psychotic phenomena, the chapter summarizes criteria from the fifth edition, text revision, of the Diagnostic and Statistical Manual of Mental Disorders and the 11th revision of the International Classification of Diseases and Related Health Problems and clarifies conceptual, terminological, and cultural issues that help sharpen the understanding of the structural factors, underlying dimensions, developmental issues, and course of psychotic disorders. A central theme concerns the multimethod assessment of symptom dimensions (including the importance of assessing insight), as well as an identification of risk and protective factors that mediate the severity of psychotic symptoms and disorders. Finally, the chapter discusses the restorative nature of interpersonal relationships in the recovery process, with implications for the role that therapeutic assessment can play in working with individuals struggling with psychotic phenomena.