Research in psychiatry could speeden its progress by taking the manifest heterogenity and variability of symptom expression within diagnostic groups more fully into account. Since Kraepelin symptom variation has been recognized to have both biological and environmental roots. If investigated systematically, such variations in illness experience will yield new subtypes that will provide greater insight into the onset, course and vulnerability of mental disorders. Studies ranging from population surveys to the laboratory have demonstrated that psychopathology is not randomly distributed in a population, nor is it constantly present in the lives of individuals. At both the population and the person levels, psychopathology varies with time and place. At the population level, illness processes localize in risk groups, who often reside in specific neighbourhoods or social settings. The individual experience of psychopathology also fluctuates with time, place and culture. This variability requires methods for case detection and treatment planning that take this into account.