This report summarizes the proceedings and recommendations of a National Heart, Lung, and Blood Institute conference on assessment methods for physical activity and physical fitness in population studies held in Bethesda, Maryland, in May, 1984. Synopses of each contributor's presentation are given, as well as specific recommendations for further research. Key areas for investigation were noted as validation and standardization of questionnaires, increased use of activity monitors in exercise research, development of simpler fitness tests, and research into possible surrogate measures for fitness evaluation.