Longitudinal studies provide the only method for directly studying the natural course of human growth and development and are therefore invaluable, both to public health and clinical medicine. However, longitudinal studies are accompanied by specific problems. The purpose of this final paper is to discuss in two separate sections some of the methodological, analytical and practical issues regarding the relevance of the findings reported. Examples of such problems and their solutions are described. Methodological issues discussed concern statistical conclusion validity, internal validity and construct validity of longitudinal studies. Practical issues discussed concern the design of longitudinal studies, the recruitment and retention of subjects, and the methods used. Finally, examples of the significance of longitudinal research outcomes to public health and clinical medicine are given.