During energy utilization and conversion, thermal energy is either an intermediate product or a byproduct; that is, thermal transport plays a critical role on efficiency, reliability, and safety of the energy systems. Recent development in nanotechnologies enabled significant improvement of the thermal energy storage performance, fuel cell, battery and thermoelectric devices. To meet the ever increasing challenges posed by energy systems, innovative and transformative measures must be taken to significantly improve the performance of these devices. Such measures will not be possible without a thorough understanding of thermal transport at molecular, nano-and microscale levels because physical phenomena occurring at the molecular, nano- and microscale will have profound effects on the performance at the system level. Understanding of multiscale thermal transport in the energy system is essential to improve their performance.