The results of the present Doctoral Thesis show that all the metabolic carts yielded different RMR and RER measures. The Omnical metabolic cart presented better accuracy and precision than the rest of metabolic carts, although the day-to-day biological reproducibility achieved by the Q-NRG and the Vyntus CPX was similar to the one achieved by the Omnical. The post-calorimetric correction procedure did not improve neither the comparability nor the day-to-day RMR and RER biological reproducibility in the four analyzed metabolic carts. On the other hand, we observed a strong association between the day-to-day biological reproducibility assessed with the CCM Express and the CPX Ultima CardiO2 metabolic carts. Finally, the longtime interval method for IC data analysis presented the best day-to-day RMR and RER biological reproducibility in four metabolic carts. Collectively, the results of this Doctoral Thesis suggest that the Omnical is the best metabolic cart for assessing RMR and RER among the six metabolic carts analyzed. Moreover, this Doctoral Thesis suggest that the day-to-day biological reproducibility is largely attributable to the individual’s characteristics and is not improved by the application of a post-calorimetric procedure based on the infusion of pure gases after the individual measurement. Finally, among the methods for IC data analysis, the long-time interval method seems to be the most adequate for analyzing the data provided by the analyzed metabolic carts.