This chapter describes light propagation in terms of modes of propagation that propagate through the optical fiber without any change in their transverse field distribution. Most fiber optic communication systems use digital communication in which information to be sent is first coded in the form of pulses and then these pulses of light are transmitted from the transmitter to the receiver where the information is decoded. The total dispersion in the case of a single-mode optical fiber occurs due to a combination of material dispersion and waveguide dispersion. The most important nonlinear effects that affect optical fiber communication systems include self-phase modulation, cross-phase modulation and four wave mixing. Stimulated Raman scattering and stimulated Brillouin scattering are also important nonlinear phenomena. Supercontinuum generation is the phenomenon in which a nearly continuous spectrally broadened output is produced through nonlinear effects on high peak power picosecond and subpicosecond pulses.