Efficient power conversion systems require passive components such as capacitors and inductors for energy storage, and switches to transfer energy into and out of these passive components. Power losses in these passives, especially magnetics, limit the use of high switching frequency to scale down their physical size and achieve miniaturization or integration of power supply on chip. The energy densities of various energy storage mechanisms are analyzed to identify opportunities for technology development for future high-frequency power conversion systems. Piezoelectric resonators and conventional electromagnetic LC resonators both show the potential to offer much higher efficiency and power density than do typical passive components in use today. Analysis of these resonators in a resonant switched-capacitor type circuit provides insight into possible technology development options.