The nature of late-onset psychosis in the absence of a dementia or secondary to organic dysfunctions in the fifth decade of life and beyond is contentious and unresolved. Different terminologies, diagnostic criteria and age cut-offs have been applied to late-onset psychosis, which have stymied clinicians and researchers. No official diagnostic designation for patients with late-onset psychosis is included in the current psychiatric diagnostic system (Diagnostic and Statistical Manual of Mental Disorders-V, International Classification of Diseases-10). The validity of this diagnostic exclusion has been questioned. Despite these problems, a relatively consistent clinical picture has reported. However, many questions remain regarding the underlying etiology, pathophysiological mechanisms, treatment and prognosis. Whether late-onset psychosis is distinct from schizophrenia and whether it might be a harbinger of dementia are unclear. Recent studies have suggested an underlying biological pathophysiology of late-onset psychosis.