This is a talk focused on the criticality of thermal systems in almost every domain of energy conversion. Thermal systems are critically important to nearly all domains of energy conversion, and controls are vital to extracting maximal efficiency from the overall system. Understanding the dynamics of transient thermal systems is the first step towards effective control design. While a great deal of understanding of steady-state performance of an overall system already exists, the combined performance of coupled and interconnected systems during transients is still not well described or understood. This becomes more important with increased system complexity or increased transient relevance. Continued improvement in control-oriented modeling will be very valuable in terms of accuracy, speed, etc. With energy as a crucial theme for a sustainable future, it is clear that the Mechanical Engineering community must play a key leadership role in achieving this potential, since the thermal energy domain is one with which we are most familiar.