The validity of any exposure assessment is an essential step in risk assessment. Following definitions of the various types of measurement validity, such as content, criterion and construct validity, and study validity, some shortcomings of actual exposure measurements are discussed. Both direct and indirect ways of measuring exposures to air, water and food pollutants are taken into consideration, with special attention being payed to indirect measurements using questionnaires and/or time-activity budgets. It is concluded that it is difficult to measure validity in an absolute way. Rather, a relative evaluation is possible comparing the validity of one result or study to that of another.