Communications can be broadly defined as the transfer of information from one point to another. In optical fibre communications, this transfer is achieved by using light as the information carrier. There has been an exponential growth in the deployment and capacity of optical fibre communication technologies and networks over the past twenty-five years. This growth has been made possible by the development of new optoelectronic technologies that can be utilised to exploit the enormous potential bandwidth of optical fibre. Today, systems are operational which operate at aggregate bit rates in excess of 100 Gb/s. Such high capacity systems exploit the optical fibre bandwidth by employing wavelength division multiplexing. Optical technology is the dominant carrier of global information. It is also central to the realisation of future networks that will have the capabilities demanded by society. These capabilities include virtually unlimited bandwidth to carry communication services of almost any kind, and full transparency that allows terminal upgrades in capacity and flexible routing of channels. Many of the advances in optical networks have been made possible by the advent of the optical amplifier.