The purpose of this chapter is to provide the means for evaluation of the performance of cogeneration and trigeneration systems from the point of view of useful energy products, primary energy consumption and primary energy savings. The correct definition and calculation of efficiencies and the primary energy savings are important in order not only to obtain a clear picture of what cogeneration can achieve but also to reveal whether a particular system is eligible for economic incentives provided in several countries for promotion of cogeneration, such as subsidy on investment, guaranties of origin and special tariff for electricity coming from high efficiency cogeneration. A cogeneration system may operate at a variety of loads and external conditions. Thus, an analysis on the design point only may lead to overestimation of its thermodynamic performance. If these results are then used for evaluation of its economic performance, they may give a wrong picture of the economic viability of the investment. For this reason, an example with off-design performance will also be presented in this chapter.