Argues that longitudinal studies can provide archives of data that can be used for many purposes in addition to such traditional ones as analyses of developmental phenomena and causal analyses. Findings from the Baltimore Longitudinal Study of Aging (N. W. Shock et al, 1984) illustrate several uses of such data, including (1) development of nomological nets for the interpretation of personality measures, (2) evaluation of state effects in personality measurement, (3) validation of retrospective reports, and (4) heuristic exploratory analyses. Current knowledge on the stability of personality in adulthood, the correspondence of observer ratings and self-reports, and the comprehensiveness of the 5-factor model can enhance the design of future longitudinal studies.