The thermodynamic analysis of the compressed-air energy storage system (CAES system) is presented. Two types of CAES systems, with respect to the underground reservoir operation, are considered: variable- and constant-pressure ones. Several configurations of the CAES system are analysed. Influence of the main thermal parameters (pressure of the stored air, aftercooling temperature, turbines intermediate pressure) on the energy performance as well as on the energy efficiency (direct and cumulative) of the CAES system are analysed.