Power electronics is a comprehensive field that integrates both academic and industrial aspects. Power electronics development relies on several key technological elements including power semiconductor devices and their implementation techniques, magnetic components, capacitors, cooling systems, circuit topologies, circuit control methods, and protective mechanisms. Power electronics must comprehensively encompass each of these technical areas to create added value. Therefore, a mutual understanding of other research fields is essential for addressing the element technologies. Magnetic devices, such as inductors and transformers, serve as fundamental components in power electronics circuits. The use of magnetic components in power converters necessitates knowledge of magnetic materials and a deep understanding of the operation of power electronics circuits. Furthermore, in power electronics circuits, the magnetic design process is crucial for meeting circuit requirements such as size, power loss, and inductance values. Generally, magnetic devices are composed of magnetic cores and windings. Designers of magnetic components aim to improve the performance of power electronics circuits by comprehensively understanding the individual characteristics of both the windings and magnetic cores. In this chapter, magnetic devices used in power electronics circuits and their fundamental design methodologies are introduced. Moreover, applied design approaches for magnetic components to achieve high power density and EMI reduction are introduced.