This chapter introduces the fundamental concepts of ferromagnetism, including the behavior of elementary magnetic moments, paramagnetism, the Weiss theory, and exchange interaction. It then presents the equation of motion for magnetization and discusses high-frequency magnetic susceptibility, including the solution of the linearized equation of motion, the peculiarities of the susceptibility tensor, and the resonant dependence of its components on frequency and magnetic field. The chapter also covers the gyrotropy of the magnetic susceptibility tensor and the resonant behavior of oscillation amplitudes under the influence of circularly polarized fields. Finally, it explores the high-frequency permeability tensor and its dependence on magnetic fields and frequencies.This chapter introduces the fundamental concepts of ferromagnetism, including the behavior of elementary magnetic moments, paramagnetism, the Weiss theory, and exchange interaction. It then presents the equation of motion for magnetization and discusses high-frequency magnetic susceptibility, including the solution of the linearized equation of motion, the peculiarities of the susceptibility tensor, and the resonant dependence of its components on frequency and magnetic field. The chapter also covers the gyrotropy of the magnetic susceptibility tensor and the resonant behavior of oscillation amplitudes under the influence of circularly polarized fields. Finally, it explores the high-frequency permeability tensor and its dependence on magnetic fields and frequencies.