Conventionally, mechanical vapour compression chillers are considered to be the most appropriate solutions for commercial and industrial air-conditioning applications. There are many categories of conventional chillers based on their cooling mechanism and operational arrangements. To evaluate their performance, numerous international and national test standards have been established. The most common efficiency parameter is coefficient of performance and it is accepted globally. On the other hand, the indirect evaporative cooler (IEC) has emerged to be an alternative air-conditioning solution but there is currently no international standard to evaluate its performance. A mathematical model is developed to evaluate the performance of both conventional chillers and indirect evaporative coolers. It showed that the indirect evaporative cooler performance is much higher (COP = 21.52 cooling only) when compared with the conventional chiller (COP = 3.26). In addition, the water consumption of the indirect evaporative cooler is also lower compared with conventional chillers. The indirect evaporative cooler can be one of the best solutions for future cooling needs. Presently, there are established performance measurement parameters for conventional chillers such as EER, SEER, COP and SCOP together with test standards for performance calculations. Unfortunately, there is no international standard available for IEC systems even though there are some regional standards employed to evaluate the performance of IEC systems. In this chapter, the details on available performance parameters are provided including all international standards. In addition, a sample calculation is provided to enable the performances of conventional chillers and IEC to be evaluated for commercial references.