Epidemiology and preventive medicine are changing together with Population and health and with ever expanding medical and non medical technologies. New technologies make epidemiology methodologically more sophisticated, but such advances risk overshadowing epidemiology's most important role: raising questions, providing answers, and helping the medical decision-making at ail levels of prevention. Epidemiology also plays a major role in the evaluation of new and other technologies whose effectiveness is poorly known. Epidemiological approaches, methods, techniques, and interpretations are widely used in new and rapidly expanding fields of medicine: research evaluation and synthesis (meta-analysis), establishment of guidelines for clinical preventive practices, new medical technology assessment, guidelines for national and international health policies, evidence-based medicine, outcomes research and disease management ('population-based' medicine and quality of care improvement). In the nearest future, infectious and noninfectious diseases may cease to be almost the sole subjects of epidemiology and they may share their place with other mass phenomena of the next millennium, such as medical practices and care, or political, social and economic actions and their consequences. Not only will primary, secondary, and tertiary Prevention will remain in the epidemiological mainstream, but health protection and health promotion will require perhaps a redefinition of epidemiology in these domains. Epidemiology and preventive medicine are both subjects of medical ethics and dilemma for right choices.