This chapter begins by giving the motivation for studying optical fibers and looks at how modern optical communication began. It then introduces some basic properties associated with the propagation of light through optical fibers using geometrical-optics description. In the mid 1970s, it was recognized that the existing copper technology would be unsuitable for future communication networks. A fiber-optic system is similar to a conventional transmission system. An optical fiber is a dielectric cylindrical waveguide operating at optical frequency. Optical frequencies are on the order of 100 THz. Some people question whether the electromagnetic radiation used in fiber should be called light since it is outside the visible band of the spectrum. The performance of a fiber-optic link depends on the numerical aperture (NA), attenuation, and dispersion characteristics of the fiber. As signals propagate through the fiber, they become distorted due to attenuation and dispersion.