The original objectives established in the mid-1960s for optical fibre systems were more than met by the first production equipment in1980. Now, second-generation single-mode systems, with still greater performance, are entering service. However, current research shows that the technology is still in its infancy. The use of highly coherent semiconductor laser transmitters coupled with heterodyne detection has given large improvements in receiver sensitivity. Understanding the nonlinear properties of glass fibres has paved the way to higher transmitter powers and the use of picosecond pulses. Many integrated-optic components such as modulators, couplers and other components will probably be used while the monolithic integration of optical and electronic components may lead to further cost reductions. The paper will describe these advances, examine their possible applications in systems and discuss how these may be exploited in the future.