It is well known that electromagnetic wave can be applied to wireless communication, radar, heating and power transmission. It has brought human society into industrial and information age. The mechanism behind these applications is electromagnetic radiation, i.e. energy transportation and conversion. Recently, research shows that evanescent wave, which is non-radiation wave associated with guided wave theory, can be amplified by metamaterials. Moreover, the sensitivity and resolution of sensor using metamaterials can be dramatically enhanced. The metamaterial-assisted sensors hope to fuel the revolution of sensing technology. This chapter aims at the study of metamaterial-assisted sensors. Firstly, traditional microwave sensors are concisely reviewed, with emphasis on the method of microwave coupling, and the working principle of resonant microwave sensor. Then, the properties of metamaterials and its application are introduced. Enhancement of sensitivity in slab waveguide with TM mode and subwavelength resolution properties of perfect slab lens are demonstrated. The mechanisms of metamaterials for amplifying evanescent wave and for improving sensitivity and resolution are studied. Thirdly, the simulation results of the microwave sensor using epsilon near-zero materials (ENZ) and metamaterial particles are given. At last, the metamaterial-assisted open resonators are discussed.