**Abstract**

Currently, high efficiency electromagnetic wave absorption plays an important role to keep away from the detection of aircraft by radar and reduce information leakage in various electronic equipment. Among the candidates of electromagnetic absorbers, ferromagnetic Ni materials possess high saturation magnetization and high permeability at high frequency (1–18 GHz), which is widely used to prepare thinner absorbing materials along with strong electromagnetic absorption properties. However, the metallic materials usually have relatively high electrical conductivity, and their permeability decreases rapidly at high frequency thanks to the eddy current losses, which is generally named as skin-depth effect. To address this issue, one effective way is to design core-shell structured Ni based composites combining magnetic cores with dielectric shells. This chapter focuses on the state-of-the-art of the microwave absorption properties of Ni-based core-shell composites, and the related electromagnetic attenuation theory about how to enhance absorption properties is also discussed in detail.

**Keywords:** ferromagnetic Ni, core-shell structure, dielectric loss, magnetic loss, impedance match
