This chapter defines psychosis as a condition involving impaired reality testing and introduces its core symptom domains: positive symptoms, negative symptoms, disorganization, and neurological soft signs. Hallucinations, delusions, and thought disorder are described with clear phenomenological distinctions and clinical examples. Negative symptoms—including avolition, anhedonia, alogia, and affective flattening—are elaborated as deficits that substantially impair functioning. This chapter also contextualizes “rational cognition” (in contrast to psychoticism) as a historically recent human development emphasizing that psychotic experience must be understood against the broader backdrop of cultural shifts. A historical overview traces the conceptual evolution from dementia praecox (premature or early dementia) to schizophrenia, the emergence of the psychosis continuum, and the modern reframing of schizophrenia as a syndrome rather than a discrete disease. Finally, this chapter summarizes major treatment modalities—pharmacotherapy, psychotherapy, rTMS, ECT, and psychosurgery—highlighting their roles and limitations in contemporary practice.