Despite many studies in the last years, obesity still needs more investigation, including its relation with energetic expenditure (EE). This manuscript consisted of a review considering: the existing techniques to measure EE, showing their advantages and limitations and modifications in EE due to obesity. EE can be evaluated by calorimetric methods, spectroscopic methods, or by questionnaires. According to the researches objectives and possibilities, all techniques present advantages and limitations. Obese individuals have been recognized as "economics" in metabolic point of view, which means that they are able to expend little energy, compared to non-obese individuals. This economy has been investigated in many aspects. They have been analyzed every component of daily energetic expenditure (resting EE, thermogenic response to feeding and EE by physical activity), and the results are controversial, mainly because of the lack on standardization of procedures and techniques. Together with evaluation of EE in humans, molecular studies have contributed to clarify many aspects. They have been identified hormones and cellular proteins located in different kinds of cells. Studies are still necessary, and it is imperative to standardize the procedures and techniques to allow the reproducibility of the results.