This paper presents an investigation of the performance indices employed in combined or multigeneration thermal systems. Specifically, the following thermal systems will be considered: (1) combined cooling and power (CCP) systems; (2) combined heating and power (CHP) systems; and (3) combined cooling, heating, and power (CCHP) systems. The investigation will highlight the main problems and limitations related to using these indices. We will propose a new procedure for evaluating the performance of CCP systems that can be generalized for use in other combined or multigeneration systems. Employing the subsystems forming any multigeneration system as a reference, the relative saving ratios of energy and exergy are calculated. These saving ratios are used as metrics for the goodness of multigeneration systems. We will also use them to calculate equivalent energetic and exergetic efficiencies of multigeneration systems. These equivalent efficiencies will be used as performance indicators of a multigeneration system as if it were producing only one of its products. The new procedure will be applied to three case studies in this paper. Results of this work indicate that the equivalent exergetic efficiency of power generation (ψP,eqv) is the most meaningful and accurate performance index for assessing the performance of multigeneration systems.