A fiber Bragg grating, with periodical perturbation of the refractive index in the fiber core, is a very attractive optical device. The fiber Bragg grating functions as a wavelength selective reflection filter in an optical line. Because of its excellent optical properties and easy integration into fiber optic systems, the fiber Bragg grating is expected to find many applications. In this article, the mechanism of refractive index changes in the fiber core, fiber Bragg grating fabrication techniques and some applications of fiber Bragg grating are described.