In order to determine the actual threshold level of habitual physical activity necessary to obtain protective effects against degenerative diseases, valid, reliable, nonreactive, and precise instruments are needed. Because regular, low-intensity exercise provides prophylactic effects, the ability to distinguish activity patterns should be a primary consideration when choosing an instrument to assess activity in the field. Although self-report methods in the form of questionnaires are the instruments of choice when large populations are to be assessed, they are somewhat limited in their objectivity. The doubly labeled water (DLW) technique is a highly accurate and objective technique for assessing total daily energy expenditure that should be utilized more as a criterion measure for validating questionnaires that assess energy expenditure. However, the DLW technique remains too costly to be considered practical for large-scale studies. Supplementing questionnaires with a personal interview does elicit more detailed data, but activity monitors provide a more objective measure of activity that can be used as an adjunct to questionnaires. Several models of activity monitors are capable of collecting and storing data for many days, weeks, or even months. More importantly, the internal real-time clocks of these monitors allow the discrimination of activity patterns. Future studies should combine the use of these monitors with a suitable questionnaire.