The study of events in time is fundamental to biomedical research and public health surveillance. Clinicians have an intuitive appreciation for the relevance of time to health and disease, and patients invariably ask questions relating to time for which clinicians need answers. An appreciation of the picture of health and disease over time is equally fundamental to public health. Research designs that incorporate time have long been in use; the cohort study, which involves follow-up of persons overtime, is the fundamental design. Analysis of longitudinal data, generated by cohort studies and related approaches, has been enhanced by new statistical methods that are appropriate to data collected over time from repeated observations. In the next millennium, new and increasingly complex questions will undoubtedly require investigation. The research designs and analytic methods used to address clinical questions in time will continue to evolve and will provide better, sharper answers.