During the past several decades, physical activity researchers have tried to search for accurate and reliable measures for physical activity in order to better understand individual's physical activity patterns and to design effective interventions. Usually, these efforts were accomplished by correlating measures with a criterion measure. A criterion measure is defined as the most valid measure or test of the characteristic being investigated. It should be the most accurate indicator of what is to be measured. VO2 max measured on a treadmill is a good example of a criterion measure for cardiovascular endurance. Inappropriate criterion measures, however, are often used to provide the validity evidence in physical activity research. Several studies have reported the validity evidence of physical activity measures correlating with other field measures (e.g., motion sensors) instead of using criterion measure (e.g., direct observation). Because the precision of the field measure is much lower than that of criterion measure, the large variations in validity coefficients are examined in the validation studies. A number of criterion measures have been used for assessing physical activity: Doubly labeled water, indirect calorimetry, diary, direct observation, etc. However, no specific method can be identified as the best method for all investigations. Each method has its own advantages and disadvantages, which must be carefully considered before selecting a measure. This presentation will provide a review of several criterion measures used in physical activity area with their strengths and weaknesses. Some critical issues in the usage of criterion measures will also be reviewed.Keyword(s): exercise/fitness, measurement/evaluation, physical activity