A range of epidemiological studies conducted over the past decades has produced strong support for the view that lack of physical activity is associated with increased risks of several chronic diseases, such as cardiovascular disease, diabetes mellitus, cancer, etc. Much is still unknown about the type and degree of activity that is required to influence the risk of specific diseases. Furthermore, physical activity can act as a confounder in relationships between other exposure variables (e.g. diet) and disease. Thus, the measurement of physical activity in epidemiological studies is of great importance. The questionnaire is the most frequently used method in epidemiological research. Before using a questionnaire on a large scale, validity and reproducibility should be assessed in a representative population. Some practical and methodological aspects of physical activity validation studies are described, together with the possible implications of the results.