Understanding the diagnostic classification of psychotic disorders requires a review of historical changes in psychiatric nosological systems. In reviewing these changes, there are multiple factors that we need to consider. These include: Tracing the conceptual changes in how psychosis has been understood over time. Noting how affective symptoms have been understood in the diagnosis of psychotic conditions. Being aware of key empirical studies in differential diagnosis. Understanding the impact of new psychotropic medications on diagnostic practice. Finally, it is essential to realize how, with the advent of the DSM-III, the shift in emphasis to interrater reliability began to shape current diagnostic practices for all conditions, including psychoses.