This chapter explains theories and technologies related to high-frequency power magnetic devices used in DC-DC converters, especially power inductors. To improve the performance of DC-DC converters and switching power supplies, which are core technologies of power electronics, a wide range of comprehensive technologies such as materials, parts, circuits, control, structure, and analysis are required. In particular, power magnetic components, which occupy a large volume in DC-DC converters, are in high demand for high performance, low loss, and small size and weight. The development and design of power magnetic components requires not only “technology” to apply scientific theory but also “skills” to apply empirical knowledge. The chapter aims to deepen knowledge and technology related to power inductors used in DC-DC converters by comparing magnetic circuits with electric circuits, emphasizing not only “technology” but also “skills.” Power electronics is an applied system technology that makes effective use of electricity. Power electronics can be said to be a technology that increases the value of electrical products and brings out the performance of electronic components. A prosperous future society will be supported by power electronics.