Thermal management and air distribution performance are assuming a key role for achieving the energy saving and the IT equipment reliability for data centers (DCs). In recent years, to monitor and to control their variation several thermal performance metrics were introduced. This work presents a critical review on the most important thermal indices for DCs currently used. The main formulas and physical models on which they are based were discussed. Moreover, a critical analysis on the main advantages and drawbacks of each metric is carried out.