This book analyzes multi-MHz high frequency resonant DC-DC power converters with operating frequencies ranging from several MHz to tens of MHz in detail, aiming to support researchers and engineers with a focus on multi-MHz high frequency converters. The inverter stage, rectifier stage, matching network stage are analyzed in detail. Based on the three basic stages, typical non-isolated and isolated resonant DC-DC converters are depicted. To reduce the high driving loss under multi-MHz, resonant driving methods are introduced and improved. Also, the design and selection methods of passive and active component under multi-MHz frequency are described, especially for aircore inductor and transformer. Furthermore, multi-MHz resonant converter provides an approach for achieving flexible system.