This review aims to examine the relationship between phase and duration of illness and the clarity and stability of syndromes in psychotic illness. Originating from clinical experience with first episode psychosis, where Neo-Kraepelinian templates and the underlying model have felt especially awkward and ill-fitting, a hypothesis that the clinical picture of functional psychosis might become clearer over time and stabilise in individual patients is proposed which involves the notion of differentiation. A broad range of psychopathological studies and historical papers which refer directly or indirectly to the evolution of the clinical picture in psychosis over time are reviewed. The findings are difficult to interpret, since few studies have focused directly on the issue; however there is evidence of evolution over time of the clinical picture, particularly within the schizophrenia spectrum, and instability beyond this. Further research is needed to clarify the longitudinal patterns of psychopathology in psychotic illness. The capacity for evolution and instability and consequent scope for reclassification of patients has important implications for research and psychoeducation in early psychosis.