Since the turn of the century and up to about 1980, there have been two generations of epidemiological studies of the true prevalence of psychiatric disorders: a pre-World War II first generation and a post-World War II second generation. With the appearance of DSM-III in 1980 and the changes in epidemiological procedures coincident with it, it has become meaningful in the US to talk about the beginnings of a new, third generation of studies in psychiatric epidemiology. The purposes of this paper are: first, to briefly summarize the problems of validity with the procedures for case identification and diagnosis in the first- and second-generation studies; second, to consider some of the newer developments with regard to diagnostic instruments that either are or should be influencing third-generation studies; third, to discuss some of the problems of validity in the handful of third-generation studies done so far; and fourth, to describe and illustrate an approach that seems to make sense in the context of gaps in knowledge of aetiology and pathogenesis that leave us still dependent on interviews for case identification and classification.