This paper details the concept of using an exergy-based method as a thermal design methodology tool for integrated aircraft thermal systems. An exergy-based approach was applied to the design of an environmental control system (ECS) of an advanced aircraft. Concurrently, a traditional energy-based approach was applied to the same system. Simplified analytical models of the ECS were developed for each method and compared to determine the validity of using the exergy approach to facilitate the design process in optimizing the overall system for a minimum gross takeoff weight (GTW). The study identified some roadblocks to assessing the value of using an exergy-based approach. Energy and exergy methods seek answers to somewhat different questions making direct comparisons awkward. Also, high entropy generating devices can dominate the design objective of the exergy approach. Nonetheless, exergy methods do provide information to aid design providing a ready estimate for efficiency on a component and system basis. The results from the two analyses did provide similar while not exact solutions. While the paper will illustrate the methodology and its implementation, further progress is necessary to validate the hypothesis that exergy-based methods are advantageous for the design of integrated systems.