In resent years, obesity has become a major public health crisis in USA. According to the report of Department of Health and Human Services (DHHS, 2003), about 64% of US adults were overweight and more than 30% were obese in 1999-2000. Obesity has been identified as the major cause of many chronic diseases. Since total energy expenditure contributes to maintenance of healthy body weight, accurate estimation of energy expenditure of physical activity becomes very important for any effective interventional programs for obesity. MET score from the Compendium of Physical Activity has been widely used to represent physical activity intensity level and to calculate and assess energy expenditure of physical activities across studies (Ainsworth, et al., 1993; 2000). However, the Compendium only provides MET intensity levels for normal healthy adults. No systematic effort was made to determine the MET scores of physical activities in the obese population. It has been believed that, because of changes in metabolic efficiency, heavier body mass and larger body fat percentage will lead to the different MET scores for the same physical activity engaged by obese and non-obese individuals. Some studies have found that the MET scores and energy expenditure are higher among obese individuals than the ones listed in the Compendium in weight-bearing activities; In non-weight-bearing activities, the opposite pattern could be observed (Howell et al., 1999; Schmitz, 1998).Research findings in this area, however, have not been consistent. A few studies found that there was no difference of MET scores between obese and non-obese individuals in low-intensity physical activities, but did exist difference in moderate and vigorous activities. This presentation will provide a detailed review of major issues of MET scoring for the obese. Current research challenges and future directions will also be addressed.