This paper outlines a newly emerging body of work that relies on exergy analysis and thermodynamic optimization in the design of energy systems for modern aircraft. Exergy analysis establishes the theoretical performance limit. The minimization of exergy destruction brings the design as closely as permissible to the theoretical limit The system architecture springs out of this constrained optimization principle. A key problem is the extraction of maximum exergy from a hot gaseous stream that is gradually cooled and eventually discharged into the ambient. The optimal configuration consists of a heat transfer surface with a temperature that decays exponentially in the flow direction. This configuration can be achieved in a counterflow heat exchanger with an optimal imbalance of flow capacity rates. The same optimal configuration emerges when the surface is minimized subject to specified exergy extraction rate. Similar opportunities for optimally matching components and streams exist in considerably more complex systems for power and refrigeration. They deserve to be pursued, and can be approached first at the conceptual level, based on exergy analysis and thermodynamic optimization. The application of such principles in aircraft energy system design also sheds light on the design principle that generated all the systems that use powered flight, engineered and natural.