The definitions and purposes of energy systems and process systems are compared in this paper by considering the role of energy in systems with emphasis on the differences in the methods used to optimize the exergoeconomic benefits of both types of systems. The process system exergoeconomic relationship primarily include the establishment of the Three-link model of process systems, the composition of objective functions, and the corresponding boundary constraint conditions for optimization. The Three-Link-Method (TLM) which is based on exergoeconomics, can be used to effectively optimize process systems. The TLM used to optimize process systems is outlined in this paper. Some aspects which need further development are also identified.