In the treatment of obesity, assessment of resting energy expenditure (REE) can provide the basis for prescribing an individualized energy intake to attain a desired level of energy deficit. Indirect calorimetry is the most frequently used method to measure REE, but the great expense of equipment precludes its widespread use. As a result, REE is often estimated by predictive formulas based on weight, height, age, and gender. This paper examines the accuracy of these formulas in estimating REE among obese patients, discusses the assumptions underlying their use, and reviews the need for technological advances that will make the assessment of REE accurate, portable, and inexpensive.