We present a review on the noise in mesoscopic physics. The main sources of noise are briefed. The fluctuations in the occupation number give rise to thermal noise, while shot noise is a consequence of the quantization of charge. We introduce the main theory of noise——scattering theory, which has been applied to saddle-shaped semiconductor quantum wires and ferromagnetic/insulator/semiconductor double heterojunctions. The physical significance of the study of noise are pointed out.