The United States is experiencing a rapid increase in the population of older people. In 2000, there were an estimated 35 million people age 65 or older in the United States, which accounted for almost 13 percent of the total population. Assessing physical activity in an accurate and reliable way is essential in determining the health status and lifestyle of older adults and in designing more effective interventions. The determination of the energy expenditure of physical activities of a person will be dependent on the MET values published in the compendium (Ainsworth et al., 2000). MET is the ratio of work metabolic rate to a standard resting metabolic rate of 1.0 (4.184 kj)•kg-1•h-1 and 1 MET is considered a resting metabolic rate obtained during quiet sitting. Published MET values, however, are based on young adults, and their values may vary in subgroups with different age, race, fitness level, mechanical efficiency, and geographic and environmental conditions. A correction factor, therefore, may be needed to adjust for individual difference when estimating the energy expenditure of physical activities in individuals, but no such correction has been developed (Ainsworth et al., 2000). In general, extensive scientific evidence has shown that individuals’ total energy expenditures (TEE) decline as they become older. After an analysis of 568 doubly labeled water measures in “healthy” participants (184 measurements in participants over age 65), Elia et al. (2000) reported there is a decrease of .69 and .43 MJ/day/decade respectively in men (standard weight 75 kg) and women (standard weight 67 kg). Thus, it seems logical to assume that when an older adult engages in a free-living physical activity, a similar decline will be lessened. If this is true, applying existing MET information in the compendium, which is based on the average information of young adults, may overestimate the energy expenditure of physical activity for older adults. The data-based research, however, is urgently needed to verify the assumption and a statistical adjustment should be able to be generated if the assumption can be confirmed. This presentation will discuss the major issues related to MET values of the aging population.